diff --git a/.gitea/workflows/cd.yml b/.gitea/workflows/cd.yml
index 10f8e9e..7e30f38 100644
--- a/.gitea/workflows/cd.yml
+++ b/.gitea/workflows/cd.yml
@@ -47,6 +47,13 @@ jobs:
           docker push $IMAGE_BASE-collab:${{ github.sha }}
           docker push $IMAGE_BASE-collab:test
 
+      - name: Build and push backup image
+        run: |
+          docker build -f apps/backup/Dockerfile --build-arg APP_VERSION=${{ github.sha }} \
+            -t $IMAGE_BASE-backup:${{ github.sha }} -t $IMAGE_BASE-backup:test .
+          docker push $IMAGE_BASE-backup:${{ github.sha }}
+          docker push $IMAGE_BASE-backup:test
+
   deploy-test:
     name: Deploy to Test
     needs: build-push
@@ -120,6 +127,7 @@ jobs:
           docker buildx imagetools create -t $IMAGE_BASE-web:int $IMAGE_BASE-web:${{ github.sha }}
           docker buildx imagetools create -t $IMAGE_BASE-api:int $IMAGE_BASE-api:${{ github.sha }}
           docker buildx imagetools create -t $IMAGE_BASE-collab:int $IMAGE_BASE-collab:${{ github.sha }}
+          docker buildx imagetools create -t $IMAGE_BASE-backup:int $IMAGE_BASE-backup:${{ github.sha }}
 
       - name: Set up SSH
         run: |
diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index 9c482ff..00dd2b0 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -538,3 +538,6 @@ jobs:
 
       - name: Build collab image
         run: docker build -f apps/collab/Dockerfile --build-arg APP_VERSION=${{ github.sha }} -t dorfteich-collab:ci .
+
+      - name: Build backup image
+        run: docker build -f apps/backup/Dockerfile --build-arg APP_VERSION=${{ github.sha }} -t dorfteich-backup:ci .
diff --git a/apps/api/src/config/secret-store.ts b/apps/api/src/config/secret-store.ts
index 88da4f3..b9ac2cb 100644
--- a/apps/api/src/config/secret-store.ts
+++ b/apps/api/src/config/secret-store.ts
@@ -2,44 +2,16 @@ import { existsSync, readFileSync } from 'node:fs';
 import { chmod, mkdir, rename, writeFile } from 'node:fs/promises';
 import { dirname } from 'node:path';
 
+import { parseSecretsFile, serializeSecrets } from '@dorfteich/shared';
+
 /**
- * The env-backed secret store (security.md §Secrets, issue #80): secrets the
- * setup wizard collects in the browser (SMTP credentials) are persisted as a
- * mode-600 dotenv-style file on a volume — never as database rows. The file
- * extends the environment: `overlayEnv` fills only variables the process
- * environment does not set, so the stage `.env` always stays authoritative.
+ * File I/O for the env-backed secret store (security.md §Secrets, issue #80).
+ * The pure format/merge helpers (`parseSecretsFile`, `serializeSecrets`,
+ * `overlayEnv`) moved to `@dorfteich/shared` in issue #83 so the backup
+ * sidecar can read the same store; they are re-exported here to keep the
+ * api-internal import paths stable.
  */
-
-/** Parses the dotenv-style store content. Ignores blank lines and comments. */
-export function parseSecretsFile(content: string): Record<string, string> {
-  const secrets: Record<string, string> = {};
-  for (const line of content.split('\n')) {
-    const trimmed = line.trim();
-    if (!trimmed || trimmed.startsWith('#')) continue;
-    const eq = trimmed.indexOf('=');
-    if (eq <= 0) continue;
-    const key = trimmed.slice(0, eq).trim();
-    let value = trimmed.slice(eq + 1).trim();
-    if (value.startsWith('"') && value.endsWith('"') && value.length >= 2) {
-      value = value.slice(1, -1).replace(/\\n/g, '\n').replace(/\\"/g, '"').replace(/\\\\/g, '\\');
-    }
-    secrets[key] = value;
-  }
-  return secrets;
-}
-
-/** Serializes secrets with double-quoted, escaped values (dotenv-compatible). */
-export function serializeSecrets(secrets: Record<string, string>): string {
-  const lines = [
-    '# Managed by Dorfteich (setup wizard). Values here fill environment',
-    '# variables that the container environment does not set explicitly.',
-  ];
-  for (const [key, value] of Object.entries(secrets)) {
-    const escaped = value.replace(/\\/g, '\\\\').replace(/"/g, '\\"').replace(/\n/g, '\\n');
-    lines.push(`${key}="${escaped}"`);
-  }
-  return lines.join('\n') + '\n';
-}
+export { overlayEnv, parseSecretsFile, serializeSecrets } from '@dorfteich/shared';
 
 /** Reads the store file; a missing file is an empty store, not an error. */
 export function readSecretsFile(path: string): Record<string, string> {
@@ -47,27 +19,6 @@ export function readSecretsFile(path: string): Record<string, string> {
   return parseSecretsFile(readFileSync(path, 'utf8'));
 }
 
-/**
- * Merges the store under the real environment: explicit env vars win, store
- * values fill the gaps (and Zod defaults fill whatever remains at parse
- * time). Empty strings count as unset on both sides — compose passes
- * `${SMTP_HOST:-}` as `""` for variables the stage `.env` does not define,
- * and those must not shadow wizard-written store values or schema defaults.
- */
-export function overlayEnv(
-  env: Record<string, string | undefined>,
-  secrets: Record<string, string>,
-): Record<string, string | undefined> {
-  const merged: Record<string, string | undefined> = {};
-  for (const [key, value] of Object.entries(secrets)) {
-    if (value !== '') merged[key] = value;
-  }
-  for (const [key, value] of Object.entries(env)) {
-    if (value !== undefined && value !== '') merged[key] = value;
-  }
-  return merged;
-}
-
 /**
  * Merges entries into the store file atomically (staging file + rename, so a
  * crash mid-write never leaves a torn file) and keeps it owner-only readable.
diff --git a/apps/backup/Dockerfile b/apps/backup/Dockerfile
new file mode 100644
index 0000000..49d7634
--- /dev/null
+++ b/apps/backup/Dockerfile
@@ -0,0 +1,31 @@
+# Build context is the repository root (workspace build):
+#   docker build -f apps/backup/Dockerfile .
+
+FROM node:22.15-alpine AS build
+WORKDIR /repo
+RUN npm install -g pnpm@11
+COPY pnpm-workspace.yaml pnpm-lock.yaml package.json tsconfig.base.json ./
+COPY packages/shared ./packages/shared
+COPY apps/backup ./apps/backup
+RUN pnpm install --frozen-lockfile --filter @dorfteich/backup... \
+  && pnpm --filter @dorfteich/shared build \
+  && pnpm --filter @dorfteich/backup build \
+  # Self-contained production bundle (prod deps only) at /out.
+  && pnpm --filter @dorfteich/backup deploy --prod --legacy /out \
+  && cp -r apps/backup/dist /out/dist
+
+FROM node:22.15-alpine
+ARG APP_VERSION=0.0.0-dev
+ENV NODE_ENV=production APP_VERSION=${APP_VERSION} \
+    # Baked-in volume paths (self-sufficient without compose env, like the
+    # api image's PLUGINS_DIR — issue #71's lesson).
+    BACKUPS_DIR=/backups UPLOADS_DIR=/data/uploads PLUGINS_DIR=/data/plugins \
+    SECRETS_FILE=/data/secrets/secrets.env
+# pg_dump/pg_restore matching the stack's postgres:17 server, GNU tar for the
+# volume archives, tzdata so BACKUP_TIME honors a configured TZ.
+RUN apk add --no-cache postgresql17-client tar tzdata \
+  && mkdir -p /backups && chown node:node /backups
+WORKDIR /app
+COPY --from=build --chown=node:node /out /app
+USER node
+CMD ["node", "dist/index.js"]
diff --git a/apps/backup/package.json b/apps/backup/package.json
new file mode 100644
index 0000000..a9b648b
--- /dev/null
+++ b/apps/backup/package.json
@@ -0,0 +1,28 @@
+{
+  "name": "@dorfteich/backup",
+  "version": "0.0.0",
+  "private": true,
+  "type": "module",
+  "description": "Dorfteich backup sidecar — nightly pg_dump + volume archive with prune, status.json, and failure mail (ADR 0015)",
+  "license": "MIT",
+  "main": "dist/index.js",
+  "scripts": {
+    "build": "tsc -p tsconfig.json",
+    "start": "node dist/index.js",
+    "start:dev": "tsx watch src/index.ts",
+    "typecheck": "tsc --noEmit",
+    "test": "vitest run --passWithNoTests"
+  },
+  "dependencies": {
+    "@dorfteich/shared": "workspace:*",
+    "nodemailer": "^9.0.3",
+    "pino": "^9.6.0",
+    "zod": "^3.25.76"
+  },
+  "devDependencies": {
+    "@types/node": "^26.1.0",
+    "@types/nodemailer": "^8.0.1",
+    "tsx": "^4.19.0",
+    "vitest": "^3.0.0"
+  }
+}
diff --git a/apps/backup/src/archive.test.ts b/apps/backup/src/archive.test.ts
new file mode 100644
index 0000000..b73b453
--- /dev/null
+++ b/apps/backup/src/archive.test.ts
@@ -0,0 +1,55 @@
+import { mkdir, mkdtemp, readFile, rm, writeFile } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+
+import { archiveBase, createArchive, extractArchive } from './archive.js';
+
+let dir: string;
+
+beforeEach(async () => {
+  dir = await mkdtemp(join(tmpdir(), 'dorfteich-archive-'));
+});
+
+afterEach(async () => {
+  await rm(dir, { recursive: true, force: true });
+});
+
+describe('archiveBase', () => {
+  it('requires a shared parent directory', () => {
+    expect(() => archiveBase(['/data/uploads', '/other/plugins'])).toThrow(/share one parent/);
+    expect(archiveBase(['/data/uploads', '/data/plugins'])).toEqual({
+      base: '/data',
+      names: ['uploads', 'plugins'],
+    });
+  });
+});
+
+describe('create/extract round-trip (real tar)', () => {
+  it('restores the archived files byte-identically', async () => {
+    const source = join(dir, 'source');
+    await mkdir(join(source, 'uploads', 'ab'), { recursive: true });
+    await mkdir(join(source, 'plugins'), { recursive: true });
+    await writeFile(join(source, 'uploads', 'ab', 'file.png'), 'png-bytes');
+    await writeFile(join(source, 'plugins', 'manifest.json'), '{"id":"toc"}');
+
+    const archive = join(dir, 'files.tar.gz');
+    await createArchive(archive, [join(source, 'uploads'), join(source, 'plugins')]);
+
+    const target = join(dir, 'target');
+    await mkdir(target, { recursive: true });
+    await extractArchive(archive, [join(target, 'uploads'), join(target, 'plugins')]);
+
+    expect(await readFile(join(target, 'uploads', 'ab', 'file.png'), 'utf8')).toBe('png-bytes');
+    expect(await readFile(join(target, 'plugins', 'manifest.json'), 'utf8')).toBe('{"id":"toc"}');
+  });
+
+  it('produces a valid empty archive when no data directory exists yet', async () => {
+    const archive = join(dir, 'empty.tar.gz');
+    await createArchive(archive, [join(dir, 'none', 'uploads'), join(dir, 'none', 'plugins')]);
+    const target = join(dir, 'extract');
+    await mkdir(target, { recursive: true });
+    await extractArchive(archive, [join(target, 'uploads'), join(target, 'plugins')]);
+  });
+});
diff --git a/apps/backup/src/archive.ts b/apps/backup/src/archive.ts
new file mode 100644
index 0000000..d762dc6
--- /dev/null
+++ b/apps/backup/src/archive.ts
@@ -0,0 +1,50 @@
+import { execFile } from 'node:child_process';
+import { existsSync } from 'node:fs';
+import { basename, dirname } from 'node:path';
+import { promisify } from 'node:util';
+
+const execFileAsync = promisify(execFile);
+
+/**
+ * The uploads and plugins directories travel in one tar archive per restore
+ * set (ADR 0015). Both must share a parent directory (in the compose stack
+ * that is `/data`): entries are stored relative to it (`uploads/…`,
+ * `plugins/…`), so extraction lands exactly where the api reads.
+ */
+export function archiveBase(dataDirs: string[]): { base: string; names: string[] } {
+  const bases = new Set(dataDirs.map((dir) => dirname(dir)));
+  if (bases.size !== 1) {
+    throw new Error(
+      `data directories must share one parent to form a single archive, got: ${dataDirs.join(', ')}`,
+    );
+  }
+  return { base: [...bases][0]!, names: dataDirs.map((dir) => basename(dir)) };
+}
+
+async function runTar(args: string[]): Promise<void> {
+  try {
+    await execFileAsync('tar', args, { maxBuffer: 16 * 1024 * 1024 });
+  } catch (error) {
+    const stderr = (error as { stderr?: string }).stderr?.trim();
+    throw new Error(`tar failed: ${stderr || (error as Error).message}`);
+  }
+}
+
+/** Creates the gzip'd volume archive; directories that do not exist yet
+ * (fresh instance without uploads) are skipped, an empty set still produces
+ * a valid empty archive. */
+export async function createArchive(outFile: string, dataDirs: string[]): Promise<void> {
+  const { base, names } = archiveBase(dataDirs);
+  const existing = names.filter((name) => existsSync(`${base}/${name}`));
+  if (existing.length === 0) {
+    await runTar(['-czf', outFile, '--files-from', '/dev/null']);
+    return;
+  }
+  await runTar(['-czf', outFile, '-C', base, ...existing]);
+}
+
+/** Unpacks a volume archive back over the data directories (restore path). */
+export async function extractArchive(archiveFile: string, dataDirs: string[]): Promise<void> {
+  const { base } = archiveBase(dataDirs);
+  await runTar(['-xzf', archiveFile, '-C', base]);
+}
diff --git a/apps/backup/src/backup-set.test.ts b/apps/backup/src/backup-set.test.ts
new file mode 100644
index 0000000..ab8a951
--- /dev/null
+++ b/apps/backup/src/backup-set.test.ts
@@ -0,0 +1,73 @@
+import { describe, expect, it } from 'vitest';
+
+import {
+  archiveFileName,
+  backupIdTime,
+  dumpFileName,
+  expiredSets,
+  listSets,
+  newBackupId,
+} from './backup-set.js';
+
+const DAY = 24 * 60 * 60 * 1000;
+
+function setFiles(id: string): string[] {
+  return [dumpFileName(id), archiveFileName(id)];
+}
+
+describe('backup ids', () => {
+  it('round-trips through the id format', () => {
+    const now = new Date('2026-07-11T03:00:00Z');
+    const id = newBackupId(now);
+    expect(id).toBe('20260711-030000');
+    expect(backupIdTime(id)?.toISOString()).toBe(now.toISOString());
+  });
+
+  it('rejects malformed ids', () => {
+    expect(backupIdTime('not-a-backup')).toBeNull();
+    expect(backupIdTime('20260711')).toBeNull();
+  });
+});
+
+describe('listSets', () => {
+  it('groups files by id and flags completeness', () => {
+    const sets = listSets([
+      'status.json',
+      'db-20260701-030000.dump.partial',
+      ...setFiles('20260710-030000'),
+      'db-20260711-030000.dump',
+    ]);
+    expect(sets.map((set) => set.id)).toEqual(['20260710-030000', '20260711-030000']);
+    expect(sets[0]?.complete).toBe(true);
+    expect(sets[1]?.complete).toBe(false);
+  });
+});
+
+describe('expiredSets', () => {
+  const now = new Date('2026-07-31T04:00:00Z');
+
+  it('removes only sets past retention', () => {
+    const fresh = newBackupId(new Date(now.getTime() - 2 * DAY));
+    const old = newBackupId(new Date(now.getTime() - 10 * DAY));
+    const newest = newBackupId(new Date(now.getTime() - 1 * DAY));
+    const sets = listSets([...setFiles(old), ...setFiles(fresh), ...setFiles(newest)]);
+
+    const expired = expiredSets(sets, now, 7);
+    expect(expired.map((set) => set.id)).toEqual([old]);
+  });
+
+  it('never removes the newest complete set, even when expired', () => {
+    const older = newBackupId(new Date(now.getTime() - 40 * DAY));
+    const newestComplete = newBackupId(new Date(now.getTime() - 20 * DAY));
+    // A newer but incomplete set (crashed run) must not shield the complete one.
+    const newerIncomplete = newBackupId(new Date(now.getTime() - 9 * DAY));
+    const sets = listSets([
+      ...setFiles(older),
+      ...setFiles(newestComplete),
+      dumpFileName(newerIncomplete),
+    ]);
+
+    const expired = expiredSets(sets, now, 7);
+    expect(expired.map((set) => set.id)).toEqual([older, newerIncomplete]);
+  });
+});
diff --git a/apps/backup/src/backup-set.ts b/apps/backup/src/backup-set.ts
new file mode 100644
index 0000000..6254134
--- /dev/null
+++ b/apps/backup/src/backup-set.ts
@@ -0,0 +1,89 @@
+/**
+ * A restore set (ADR 0015) is one nightly `pg_dump` plus the matching
+ * uploads/plugins archive, tied together by a shared backup id derived from
+ * the run's UTC start time. This module owns the naming scheme and the pure
+ * prune decision; the runner applies it to the filesystem.
+ */
+
+export interface BackupSet {
+  id: string;
+  /** File names (not paths) present in the backups directory. */
+  files: string[];
+  /** Complete = both the dump and the volume archive exist. */
+  complete: boolean;
+}
+
+const ID_PATTERN = /^(\d{4})(\d{2})(\d{2})-(\d{2})(\d{2})(\d{2})$/;
+const SET_FILE_PATTERN = /^(?:db-|files-)(\d{8}-\d{6})\.(?:dump|tar\.gz)$/;
+
+/** Backup id for a run starting now: UTC timestamp, filesystem-safe. */
+export function newBackupId(now: Date): string {
+  const pad = (value: number): string => String(value).padStart(2, '0');
+  return (
+    `${now.getUTCFullYear()}${pad(now.getUTCMonth() + 1)}${pad(now.getUTCDate())}` +
+    `-${pad(now.getUTCHours())}${pad(now.getUTCMinutes())}${pad(now.getUTCSeconds())}`
+  );
+}
+
+/** The UTC time encoded in a backup id, or null for a malformed id. */
+export function backupIdTime(id: string): Date | null {
+  const match = ID_PATTERN.exec(id);
+  if (!match) return null;
+  const [, year, month, day, hour, minute, second] = match;
+  return new Date(
+    Date.UTC(
+      Number(year),
+      Number(month) - 1,
+      Number(day),
+      Number(hour),
+      Number(minute),
+      Number(second),
+    ),
+  );
+}
+
+export function dumpFileName(id: string): string {
+  return `db-${id}.dump`;
+}
+
+export function archiveFileName(id: string): string {
+  return `files-${id}.tar.gz`;
+}
+
+/**
+ * Groups the backup directory's file names into sets, oldest first. Files
+ * that do not belong to the naming scheme (status.json, `.partial` staging
+ * files of a running or crashed run) are ignored — prune never touches them.
+ */
+export function listSets(fileNames: string[]): BackupSet[] {
+  const byId = new Map<string, string[]>();
+  for (const name of fileNames) {
+    const match = SET_FILE_PATTERN.exec(name);
+    if (!match || !backupIdTime(match[1]!)) continue;
+    const files = byId.get(match[1]!) ?? [];
+    files.push(name);
+    byId.set(match[1]!, files);
+  }
+  return [...byId.entries()]
+    .sort(([a], [b]) => a.localeCompare(b))
+    .map(([id, files]) => ({
+      id,
+      files: files.sort(),
+      complete: files.includes(dumpFileName(id)) && files.includes(archiveFileName(id)),
+    }));
+}
+
+/**
+ * The sets prune may delete: older than the retention cutoff — but never
+ * the newest complete set, even when it is expired. A stalled instance must
+ * always keep one restorable set (issue #83 acceptance criteria).
+ */
+export function expiredSets(sets: BackupSet[], now: Date, retentionDays: number): BackupSet[] {
+  const cutoff = now.getTime() - retentionDays * 24 * 60 * 60 * 1000;
+  const newestComplete = [...sets].reverse().find((set) => set.complete);
+  return sets.filter((set) => {
+    if (set === newestComplete) return false;
+    const time = backupIdTime(set.id);
+    return time !== null && time.getTime() < cutoff;
+  });
+}
diff --git a/apps/backup/src/config.test.ts b/apps/backup/src/config.test.ts
new file mode 100644
index 0000000..b75dc7d
--- /dev/null
+++ b/apps/backup/src/config.test.ts
@@ -0,0 +1,56 @@
+import { mkdtemp, rm, writeFile } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+
+import { loadBackupEnv } from './config.js';
+
+const DATABASE_URL = 'postgresql://dorfteich:pw@db:5432/dorfteich';
+
+let dir: string;
+
+beforeEach(async () => {
+  dir = await mkdtemp(join(tmpdir(), 'dorfteich-backup-env-'));
+});
+
+afterEach(async () => {
+  await rm(dir, { recursive: true, force: true });
+});
+
+describe('loadBackupEnv', () => {
+  it('applies defaults and drops compose empty strings', () => {
+    // Compose passes optional variables as "" — they must fall through to
+    // the schema defaults, not fail enum/number parsing (issue #80 lesson).
+    const env = loadBackupEnv({
+      DATABASE_URL,
+      BACKUP_TIME: '',
+      BACKUP_RETENTION_DAYS: '',
+      BACKUP_MAIL_LOCALE: '',
+      SECRETS_FILE: join(dir, 'missing.env'),
+    });
+    expect(env.BACKUP_TIME).toBe('03:00');
+    expect(env.BACKUP_RETENTION_DAYS).toBe(30);
+    expect(env.BACKUP_MAIL_LOCALE).toBe('en');
+  });
+
+  it('fills SMTP settings from the wizard-written secret store', () => {
+    const secretsFile = join(dir, 'secrets.env');
+    return writeFile(secretsFile, 'SMTP_HOST="relay.example.com"\nSMTP_PORT="2525"\n').then(() => {
+      const env = loadBackupEnv({
+        DATABASE_URL,
+        SECRETS_FILE: secretsFile,
+        // Explicit container env must win over the store.
+        SMTP_PORT: '465',
+      });
+      expect(env.SMTP_HOST).toBe('relay.example.com');
+      expect(env.SMTP_PORT).toBe(465);
+    });
+  });
+
+  it('rejects a malformed BACKUP_TIME', () => {
+    expect(() => loadBackupEnv({ DATABASE_URL, BACKUP_TIME: '25:99' })).toThrow(
+      /BACKUP_TIME.*HH:MM/s,
+    );
+  });
+});
diff --git a/apps/backup/src/config.ts b/apps/backup/src/config.ts
new file mode 100644
index 0000000..18e4abb
--- /dev/null
+++ b/apps/backup/src/config.ts
@@ -0,0 +1,24 @@
+import { existsSync, readFileSync } from 'node:fs';
+
+import {
+  backupEnvSchema,
+  overlayEnv,
+  parseEnv,
+  parseSecretsFile,
+  type BackupEnv,
+} from '@dorfteich/shared';
+
+/**
+ * Loads the sidecar configuration the same way the api does (issue #80):
+ * the wizard-written secret store fills SMTP variables the container env
+ * does not set, and empty strings count as unset — compose passes optional
+ * variables as `""`. Without this, a wizard-configured relay would never
+ * reach the failure mail.
+ */
+export function loadBackupEnv(env: Record<string, string | undefined> = process.env): BackupEnv {
+  const secretsFile = backupEnvSchema.shape.SECRETS_FILE.parse(env.SECRETS_FILE || undefined);
+  const secrets = existsSync(secretsFile)
+    ? parseSecretsFile(readFileSync(secretsFile, 'utf8'))
+    : {};
+  return parseEnv(backupEnvSchema, overlayEnv(env, secrets));
+}
diff --git a/apps/backup/src/index.ts b/apps/backup/src/index.ts
new file mode 100644
index 0000000..7eae44d
--- /dev/null
+++ b/apps/backup/src/index.ts
@@ -0,0 +1,49 @@
+import { pino } from 'pino';
+
+import { createArchive } from './archive.js';
+import { loadBackupEnv } from './config.js';
+import { sendFailureMail } from './mail.js';
+import { pgDump } from './pg.js';
+import { runBackup } from './runner.js';
+import { scheduleDaily } from './scheduler.js';
+import type { RunnerDeps } from './runner.js';
+
+/**
+ * Sidecar entrypoint: runs the nightly backup at BACKUP_TIME (ADR 0015).
+ * `BACKUP_RUN_ONCE=1` runs a single backup and exits with the outcome as
+ * the exit code — the on-demand path (`docker compose run backup`) and what
+ * the admin panel's manual trigger (#86) will call.
+ */
+const env = loadBackupEnv();
+const log = pino({ level: env.LOG_LEVEL, base: { service: 'backup' } });
+
+const deps: RunnerDeps = {
+  backupsDir: env.BACKUPS_DIR,
+  retentionDays: env.BACKUP_RETENTION_DAYS,
+  now: () => new Date(),
+  dump: (outFile) => pgDump(env.DATABASE_URL, outFile),
+  archive: (outFile) => createArchive(outFile, [env.UPLOADS_DIR, env.PLUGINS_DIR]),
+  onFailure: async (run) => {
+    const sent = await sendFailureMail(env, run);
+    if (!sent) log.warn({ backupId: run.backupId }, 'no BACKUP_MAIL_TO configured, alert not sent');
+  },
+  log,
+};
+
+if (process.env.BACKUP_RUN_ONCE === '1') {
+  const status = await runBackup(deps);
+  process.exit(status.lastRun.outcome === 'succeeded' ? 0 : 1);
+}
+
+log.info(
+  { time: env.BACKUP_TIME, retentionDays: env.BACKUP_RETENTION_DAYS, dir: env.BACKUPS_DIR },
+  'backup sidecar started',
+);
+const schedule = scheduleDaily(env.BACKUP_TIME, async () => void (await runBackup(deps)), log);
+
+for (const signal of ['SIGTERM', 'SIGINT'] as const) {
+  process.on(signal, () => {
+    schedule.stop();
+    process.exit(0);
+  });
+}
diff --git a/apps/backup/src/mail.test.ts b/apps/backup/src/mail.test.ts
new file mode 100644
index 0000000..86b5020
--- /dev/null
+++ b/apps/backup/src/mail.test.ts
@@ -0,0 +1,30 @@
+import { describe, expect, it } from 'vitest';
+
+import { renderFailureMail } from './mail.js';
+
+const input = {
+  backupId: '20260711-030000',
+  error: 'pg_dump failed: connection refused',
+  lastSuccessAt: '2026-07-10T03:00:12.000Z',
+};
+
+describe('renderFailureMail', () => {
+  it('renders the English alert with all details interpolated', () => {
+    const mail = renderFailureMail(input, 'en', 'dorfteich-test');
+    expect(mail.subject).toBe('[dorfteich-test] Backup failed (20260711-030000)');
+    expect(mail.text).toContain('Backup id: 20260711-030000');
+    expect(mail.text).toContain('Error: pg_dump failed: connection refused');
+    expect(mail.text).toContain('Last successful backup: 2026-07-10T03:00:12.000Z');
+  });
+
+  it('renders the German alert', () => {
+    const mail = renderFailureMail(input, 'de', 'dorfteich-test');
+    expect(mail.subject).toBe('[dorfteich-test] Backup fehlgeschlagen (20260711-030000)');
+    expect(mail.text).toContain('Fehler: pg_dump failed: connection refused');
+  });
+
+  it('states when no backup ever succeeded', () => {
+    const mail = renderFailureMail({ ...input, lastSuccessAt: null }, 'en', 'Dorfteich');
+    expect(mail.text).toContain('Last successful backup: none yet');
+  });
+});
diff --git a/apps/backup/src/mail.ts b/apps/backup/src/mail.ts
new file mode 100644
index 0000000..1fff616
--- /dev/null
+++ b/apps/backup/src/mail.ts
@@ -0,0 +1,93 @@
+import { createTransport } from 'nodemailer';
+
+import type { BackupEnv } from '@dorfteich/shared';
+
+import deMails from '@dorfteich/shared/i18n/de/mails.json' with { type: 'json' };
+import enMails from '@dorfteich/shared/i18n/en/mails.json' with { type: 'json' };
+
+/**
+ * The failure alert goes out through nodemailer directly instead of the
+ * api's mail outbox — when backups fail, the api may be the broken part
+ * (issue #83). Texts live in the shared `mails` i18n namespace (ADR 0012,
+ * de + en); the operator picks the language via BACKUP_MAIL_LOCALE. The
+ * catalog uses i18next's `{{var}}` syntax, interpolated here without
+ * pulling i18next into the sidecar.
+ */
+
+export interface FailureMailInput {
+  backupId: string;
+  error: string;
+  lastSuccessAt: string | null;
+}
+
+export interface RenderedFailureMail {
+  subject: string;
+  text: string;
+}
+
+const CATALOGS = { de: deMails, en: enMails } as const;
+
+function t(
+  locale: 'de' | 'en',
+  key: keyof (typeof CATALOGS)['en']['backupFailed'],
+  params: Record<string, string> = {},
+): string {
+  let text: string = CATALOGS[locale].backupFailed[key];
+  for (const [name, value] of Object.entries(params)) {
+    text = text.replaceAll(`{{${name}}}`, value);
+  }
+  return text;
+}
+
+export function renderFailureMail(
+  input: FailureMailInput,
+  locale: 'de' | 'en',
+  instanceLabel: string,
+): RenderedFailureMail {
+  const lastSuccess = input.lastSuccessAt
+    ? t(locale, 'lastSuccess', { finishedAt: input.lastSuccessAt })
+    : t(locale, 'lastSuccessNever');
+  return {
+    subject: t(locale, 'subject', { instance: instanceLabel, backupId: input.backupId }),
+    text: [
+      t(locale, 'intro', { instance: instanceLabel }),
+      '',
+      t(locale, 'backupId', { backupId: input.backupId }),
+      t(locale, 'error', { error: input.error }),
+      lastSuccess,
+      '',
+      t(locale, 'hint'),
+    ].join('\n'),
+  };
+}
+
+/** Sends the alert; returns false (after logging upstream) when no relay or
+ * recipient is configured — a missing mail must never fail the run. */
+export async function sendFailureMail(env: BackupEnv, input: FailureMailInput): Promise<boolean> {
+  if (!env.BACKUP_MAIL_TO) return false;
+  const mail = renderFailureMail(
+    input,
+    env.BACKUP_MAIL_LOCALE,
+    env.BACKUP_INSTANCE_LABEL || 'Dorfteich',
+  );
+  const transport = createTransport({
+    host: env.SMTP_HOST,
+    port: env.SMTP_PORT,
+    secure: env.SMTP_SECURE,
+    auth: env.SMTP_USER ? { user: env.SMTP_USER, pass: env.SMTP_PASS } : undefined,
+    connectionTimeout: 10_000,
+    greetingTimeout: 10_000,
+    socketTimeout: 20_000,
+  });
+  try {
+    await transport.sendMail({
+      from: env.SMTP_FROM,
+      to: env.BACKUP_MAIL_TO,
+      subject: mail.subject,
+      text: mail.text,
+    });
+    return true;
+  } finally {
+    transport.close();
+  }
+}
diff --git a/apps/backup/src/pg.ts b/apps/backup/src/pg.ts
new file mode 100644
index 0000000..1ec49c7
--- /dev/null
+++ b/apps/backup/src/pg.ts
@@ -0,0 +1,54 @@
+import { execFile } from 'node:child_process';
+import { promisify } from 'node:util';
+
+const execFileAsync = promisify(execFile);
+
+/**
+ * Connection settings for the libpq CLI tools as environment variables.
+ * Deliberately not `--dbname=<url>`: the URL carries the password, and argv
+ * is world-readable inside the container (`/proc/<pid>/cmdline`) — the
+ * credential-handling rule is env/file only, never argv.
+ */
+export function pgEnvFromUrl(databaseUrl: string): Record<string, string> {
+  const url = new URL(databaseUrl);
+  const env: Record<string, string> = {
+    PGHOST: url.hostname,
+    PGDATABASE: decodeURIComponent(url.pathname.replace(/^\//, '')),
+  };
+  if (url.port) env.PGPORT = url.port;
+  if (url.username) env.PGUSER = decodeURIComponent(url.username);
+  if (url.password) env.PGPASSWORD = decodeURIComponent(url.password);
+  return env;
+}
+
+async function runPgTool(tool: string, args: string[], databaseUrl: string): Promise<void> {
+  try {
+    await execFileAsync(tool, args, {
+      env: { ...process.env, ...pgEnvFromUrl(databaseUrl) },
+      maxBuffer: 16 * 1024 * 1024,
+    });
+  } catch (error) {
+    const stderr = (error as { stderr?: string }).stderr?.trim();
+    throw new Error(`${tool} failed: ${stderr || (error as Error).message}`);
+  }
+}
+
+/** `pg_dump -Fc` of the whole database into `outFile` (ADR 0015). */
+export async function pgDump(databaseUrl: string, outFile: string): Promise<void> {
+  await runPgTool('pg_dump', ['--format=custom', '--file', outFile], databaseUrl);
+}
+
+/**
+ * Restores a custom-format dump into the live database. `--clean
+ * --if-exists` drops recreated objects first, so the restore lands on a
+ * database that may still hold newer state (the runbook stops the app
+ * services, not the db container).
+ */
+export async function pgRestore(databaseUrl: string, dumpFile: string): Promise<void> {
+  const database = pgEnvFromUrl(databaseUrl).PGDATABASE ?? '';
+  await runPgTool(
+    'pg_restore',
+    ['--clean', '--if-exists', '--no-owner', '--dbname', database, dumpFile],
+    databaseUrl,
+  );
+}
diff --git a/apps/backup/src/restore.ts b/apps/backup/src/restore.ts
new file mode 100644
index 0000000..6f99582
--- /dev/null
+++ b/apps/backup/src/restore.ts
@@ -0,0 +1,40 @@
+import { existsSync } from 'node:fs';
+import { join } from 'node:path';
+
+import { pino } from 'pino';
+
+import { extractArchive } from './archive.js';
+import { archiveFileName, backupIdTime, dumpFileName } from './backup-set.js';
+import { loadBackupEnv } from './config.js';
+import { pgRestore } from './pg.js';
+
+/**
+ * In-container half of the restore runbook (operations.md §Backup & restore):
+ * `pg_restore --clean --if-exists` of the set's dump, then the volume archive
+ * back over the uploads/plugins mounts. The host-side `restore.sh` wraps this
+ * with stopping/starting the app services — run through that, not directly,
+ * unless you know the api and collab are down.
+ */
+const env = loadBackupEnv();
+const log = pino({ level: env.LOG_LEVEL, base: { service: 'backup-restore' } });
+
+const backupId = process.argv[2];
+if (!backupId || !backupIdTime(backupId)) {
+  log.error({ backupId }, 'usage: node dist/restore.js <backup-id> (e.g. 20260711-030000)');
+  process.exit(2);
+}
+
+const dumpFile = join(env.BACKUPS_DIR, dumpFileName(backupId));
+const archiveFile = join(env.BACKUPS_DIR, archiveFileName(backupId));
+for (const file of [dumpFile, archiveFile]) {
+  if (!existsSync(file)) {
+    log.error({ file }, 'restore set is incomplete — file not found');
+    process.exit(2);
+  }
+}
+
+log.info({ backupId }, 'restoring database dump');
+await pgRestore(env.DATABASE_URL, dumpFile);
+log.info({ backupId }, 'restoring uploads/plugins archive');
+await extractArchive(archiveFile, [env.UPLOADS_DIR, env.PLUGINS_DIR]);
+log.info({ backupId }, 'restore complete — start the stack and verify /readyz');
diff --git a/apps/backup/src/runner.test.ts b/apps/backup/src/runner.test.ts
new file mode 100644
index 0000000..e1220cc
--- /dev/null
+++ b/apps/backup/src/runner.test.ts
@@ -0,0 +1,139 @@
+import { mkdtemp, readdir, rm, writeFile } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+
+import { archiveFileName, dumpFileName } from './backup-set.js';
+import { runBackup, type RunnerDeps } from './runner.js';
+import { readStatus } from './status.js';
+
+/**
+ * Time-accelerated nightly runs (issue #83 acceptance criteria): the clock
+ * is injected, so "a month of nights" is a loop, with real files in a temp
+ * backups directory and dump/archive doubles standing in for pg_dump/tar
+ * (their real counterparts are covered by pg.ts/archive.ts on the stage).
+ */
+
+const DAY = 24 * 60 * 60 * 1000;
+const START = new Date('2026-07-01T03:00:00Z').getTime();
+
+let dir: string;
+let clock: Date;
+let failures: Array<{ backupId: string; error: string; lastSuccessAt: string | null }>;
+
+function makeDeps(overrides: Partial<RunnerDeps> = {}): RunnerDeps {
+  return {
+    backupsDir: dir,
+    retentionDays: 7,
+    now: () => new Date(clock),
+    dump: (outFile) => writeFile(outFile, 'dump-bytes'),
+    archive: (outFile) => writeFile(outFile, 'archive-bytes!'),
+    onFailure: async (run) => {
+      failures.push(run);
+    },
+    log: { info: () => {}, error: () => {} },
+    ...overrides,
+  };
+}
+
+beforeEach(async () => {
+  dir = await mkdtemp(join(tmpdir(), 'dorfteich-backup-'));
+  clock = new Date(START);
+  failures = [];
+});
+
+afterEach(async () => {
+  await rm(dir, { recursive: true, force: true });
+});
+
+describe('runBackup', () => {
+  it('produces a dump, a matching archive, and status.json', async () => {
+    const status = await runBackup(makeDeps());
+
+    const names = await readdir(dir);
+    expect(names).toContain(dumpFileName('20260701-030000'));
+    expect(names).toContain(archiveFileName('20260701-030000'));
+    expect(status.lastRun.outcome).toBe('succeeded');
+    expect(status.lastRun.sizes).toEqual({ dumpBytes: 10, archiveBytes: 14 });
+    expect(status.lastSuccess?.backupId).toBe('20260701-030000');
+    expect(readStatus(dir)).toEqual(status);
+  });
+
+  it('keeps exactly the retention window across simulated nights', async () => {
+    for (let night = 0; night < 30; night += 1) {
+      clock = new Date(START + night * DAY);
+      await runBackup(makeDeps());
+    }
+
+    const names = (await readdir(dir)).filter((name) => name.startsWith('db-'));
+    // Retention 7 days: the last 7 nights survive plus the current night's set.
+    expect(names.length).toBe(8);
+    expect(names).toContain(dumpFileName('20260730-030000'));
+    expect(names).not.toContain(dumpFileName('20260722-030000'));
+    expect(names).toContain(dumpFileName('20260723-030000'));
+  });
+
+  it('records a failed run, keeps lastSuccess, alerts, and cleans partials', async () => {
+    await runBackup(makeDeps());
+
+    clock = new Date(START + DAY);
+    const status = await runBackup(
+      makeDeps({
+        dump: async () => {
+          throw new Error('connection refused');
+        },
+      }),
+    );
+
+    expect(status.lastRun.outcome).toBe('failed');
+    expect(status.lastRun.error).toContain('connection refused');
+    expect(status.lastSuccess?.backupId).toBe('20260701-030000');
+    expect(failures).toHaveLength(1);
+    expect(failures[0]?.lastSuccessAt).toBe(new Date(START).toISOString());
+
+    const names = await readdir(dir);
+    expect(names.filter((name) => name.endsWith('.partial'))).toHaveLength(0);
+    // The last good set survives a failed night.
+    expect(names).toContain(dumpFileName('20260701-030000'));
+  });
+
+  it('a failure alert that itself fails never breaks the run', async () => {
+    const status = await runBackup(
+      makeDeps({
+        dump: async () => {
+          throw new Error('disk full');
+        },
+        onFailure: async () => {
+          throw new Error('smtp down');
+        },
+      }),
+    );
+    expect(status.lastRun.outcome).toBe('failed');
+    expect(readStatus(dir)?.lastRun.error).toContain('disk full');
+  });
+
+  it('after long downtime the newest complete set survives pruning', async () => {
+    await runBackup(makeDeps());
+
+    // 60 days later (retention 7): the old set is expired but must survive
+    // as the newest complete one until a new success replaces it.
+    clock = new Date(START + 60 * DAY);
+    const status = await runBackup(
+      makeDeps({
+        dump: async () => {
+          throw new Error('db gone');
+        },
+      }),
+    );
+    expect(status.lastRun.outcome).toBe('failed');
+    expect(await readdir(dir)).toContain(dumpFileName('20260701-030000'));
+
+    // The next success prunes it.
+    clock = new Date(START + 61 * DAY);
+    await runBackup(makeDeps());
+    const names = await readdir(dir);
+    expect(names).not.toContain(dumpFileName('20260701-030000'));
+    expect(names).toContain(dumpFileName('20260831-030000'));
+  });
+});
diff --git a/apps/backup/src/runner.ts b/apps/backup/src/runner.ts
new file mode 100644
index 0000000..fd0e5ff
--- /dev/null
+++ b/apps/backup/src/runner.ts
@@ -0,0 +1,132 @@
+import { mkdir, readdir, rename, rm, stat } from 'node:fs/promises';
+import { join } from 'node:path';
+
+import { archiveFileName, dumpFileName, expiredSets, listSets, newBackupId } from './backup-set.js';
+import {
+  readStatus,
+  writeStatus,
+  type BackupRun,
+  type BackupSizes,
+  type BackupStatus,
+} from './status.js';
+
+/**
+ * One nightly run (ADR 0015): dump first, then the volume archive (files may
+ * be minutes newer than the dump — documented consistency model), then prune,
+ * then `status.json`. Artifacts are written under a `.partial` suffix and
+ * renamed on completion, so a crash never leaves a file that looks like a
+ * restorable artifact, and prune (which ignores `.partial`) never counts a
+ * torn set as the "newest complete" one.
+ */
+
+export interface RunnerDeps {
+  backupsDir: string;
+  retentionDays: number;
+  now(): Date;
+  /** Real implementation: pg_dump -Fc (pg.ts). */
+  dump(outFile: string): Promise<void>;
+  /** Real implementation: tar of the uploads/plugins mounts (archive.ts). */
+  archive(outFile: string): Promise<void>;
+  /** Failure alert (mail.ts); errors here are logged, never rethrown. */
+  onFailure(run: { backupId: string; error: string; lastSuccessAt: string | null }): Promise<void>;
+  log: {
+    info(details: object, message: string): void;
+    error(details: object, message: string): void;
+  };
+}
+
+export async function runBackup(deps: RunnerDeps): Promise<BackupStatus> {
+  const startedAt = deps.now();
+  const backupId = newBackupId(startedAt);
+  const previous = readStatus(deps.backupsDir);
+  const lastSuccess = previous?.lastSuccess ?? null;
+  await mkdir(deps.backupsDir, { recursive: true });
+
+  const dumpFile = join(deps.backupsDir, dumpFileName(backupId));
+  const archiveFile = join(deps.backupsDir, archiveFileName(backupId));
+
+  let run: BackupRun;
+  let success: BackupStatus['lastSuccess'] = lastSuccess;
+  try {
+    const sizes = await produceArtifacts(deps, dumpFile, archiveFile);
+    const finishedAt = deps.now();
+    run = {
+      backupId,
+      startedAt: startedAt.toISOString(),
+      finishedAt: finishedAt.toISOString(),
+      durationMs: finishedAt.getTime() - startedAt.getTime(),
+      outcome: 'succeeded',
+      sizes,
+    };
+    success = { backupId, finishedAt: run.finishedAt, sizes };
+    deps.log.info({ backupId, sizes }, 'backup run succeeded');
+  } catch (error) {
+    await removePartials(deps.backupsDir);
+    const message = error instanceof Error ? error.message : String(error);
+    const finishedAt = deps.now();
+    run = {
+      backupId,
+      startedAt: startedAt.toISOString(),
+      finishedAt: finishedAt.toISOString(),
+      durationMs: finishedAt.getTime() - startedAt.getTime(),
+      outcome: 'failed',
+      error: message,
+    };
+    deps.log.error({ backupId, error: message }, 'backup run failed');
+    try {
+      await deps.onFailure({
+        backupId,
+        error: message,
+        lastSuccessAt: lastSuccess?.finishedAt ?? null,
+      });
+    } catch (mailError) {
+      deps.log.error({ backupId, error: String(mailError) }, 'failure alert could not be sent');
+    }
+  }
+
+  await prune(deps);
+
+  const status: BackupStatus = {
+    schemaVersion: 1,
+    updatedAt: deps.now().toISOString(),
+    retentionDays: deps.retentionDays,
+    lastRun: run,
+    lastSuccess: success,
+  };
+  await writeStatus(deps.backupsDir, status);
+  return status;
+}
+
+/** Dump, then archive — each staged as `.partial` and renamed only when done. */
+async function produceArtifacts(
+  deps: RunnerDeps,
+  dumpFile: string,
+  archiveFile: string,
+): Promise<BackupSizes> {
+  await deps.dump(`${dumpFile}.partial`);
+  await rename(`${dumpFile}.partial`, dumpFile);
+  await deps.archive(`${archiveFile}.partial`);
+  await rename(`${archiveFile}.partial`, archiveFile);
+  return {
+    dumpBytes: (await stat(dumpFile)).size,
+    archiveBytes: (await stat(archiveFile)).size,
+  };
+}
+
+/** Leftover staging files: from this failed run or an earlier crash — no
+ * resumption path exists, so they are dead weight either way. */
+async function removePartials(backupsDir: string): Promise<void> {
+  for (const name of await readdir(backupsDir)) {
+    if (name.endsWith('.partial')) await rm(join(backupsDir, name), { force: true });
+  }
+}
+
+async function prune(deps: RunnerDeps): Promise<void> {
+  const names = await readdir(deps.backupsDir);
+  for (const set of expiredSets(listSets(names), deps.now(), deps.retentionDays)) {
+    for (const file of set.files) {
+      await rm(join(deps.backupsDir, file), { force: true });
+    }
+    deps.log.info({ backupId: set.id }, 'pruned expired backup set');
+  }
+}
diff --git a/apps/backup/src/scheduler.test.ts b/apps/backup/src/scheduler.test.ts
new file mode 100644
index 0000000..ba36a38
--- /dev/null
+++ b/apps/backup/src/scheduler.test.ts
@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'vitest';
+
+import { msUntilNext } from './scheduler.js';
+
+// msUntilNext works in local time (the container's TZ); the tests build
+// their expectations with local-time Dates, so they hold in any zone.
+describe('msUntilNext', () => {
+  it('targets today when the time is still ahead', () => {
+    const now = new Date(2026, 6, 11, 1, 30, 0);
+    expect(msUntilNext(now, '03:00')).toBe(90 * 60 * 1000);
+  });
+
+  it('targets tomorrow when the time has passed', () => {
+    const now = new Date(2026, 6, 11, 3, 0, 1);
+    expect(msUntilNext(now, '03:00')).toBe(24 * 60 * 60 * 1000 - 1000);
+  });
+
+  it('an exact hit schedules a full day ahead (never a zero delay)', () => {
+    const now = new Date(2026, 6, 11, 3, 0, 0);
+    expect(msUntilNext(now, '03:00')).toBe(24 * 60 * 60 * 1000);
+  });
+});
diff --git a/apps/backup/src/scheduler.ts b/apps/backup/src/scheduler.ts
new file mode 100644
index 0000000..5ddf231
--- /dev/null
+++ b/apps/backup/src/scheduler.ts
@@ -0,0 +1,32 @@
+/**
+ * Daily scheduling without a cron dependency: one `setTimeout` to the next
+ * HH:MM occurrence (container-local time via TZ), re-armed after every run.
+ * A run that crosses midnight simply shifts the next one — backups are
+ * hours apart, drift by seconds is irrelevant.
+ */
+
+/** Milliseconds from `now` to the next local-time occurrence of `time` (HH:MM). */
+export function msUntilNext(now: Date, time: string): number {
+  const [hours = 0, minutes = 0] = time.split(':').map(Number);
+  const next = new Date(now);
+  next.setHours(hours, minutes, 0, 0);
+  if (next.getTime() <= now.getTime()) next.setDate(next.getDate() + 1);
+  return next.getTime() - now.getTime();
+}
+
+export function scheduleDaily(
+  time: string,
+  task: () => Promise<void>,
+  log: { info(details: object, message: string): void },
+): { stop(): void } {
+  let timer: NodeJS.Timeout;
+  const arm = (): void => {
+    const delay = msUntilNext(new Date(), time);
+    log.info({ nextRunInMs: delay }, 'next backup run scheduled');
+    timer = setTimeout(() => {
+      void task().finally(arm);
+    }, delay);
+  };
+  arm();
+  return { stop: () => clearTimeout(timer) };
+}
diff --git a/apps/backup/src/status.ts b/apps/backup/src/status.ts
new file mode 100644
index 0000000..07438bc
--- /dev/null
+++ b/apps/backup/src/status.ts
@@ -0,0 +1,57 @@
+import { existsSync, readFileSync } from 'node:fs';
+import { rename, writeFile } from 'node:fs/promises';
+import { join } from 'node:path';
+
+/**
+ * `status.json` on the backups volume is the machine-readable outcome of the
+ * last run (ADR 0015): the api's backup-freshness readiness check (#85) and
+ * the admin panel's backup card (#86) consume it. Keep the shape additive —
+ * bump `schemaVersion` on breaking changes.
+ */
+
+export const STATUS_FILE = 'status.json';
+
+export interface BackupSizes {
+  dumpBytes: number;
+  archiveBytes: number;
+}
+
+export interface BackupRun {
+  backupId: string;
+  startedAt: string;
+  finishedAt: string;
+  durationMs: number;
+  outcome: 'succeeded' | 'failed';
+  /** Present on failure: the first error the run hit, as a plain string. */
+  error?: string;
+  /** Present on success. */
+  sizes?: BackupSizes;
+}
+
+export interface BackupStatus {
+  schemaVersion: 1;
+  updatedAt: string;
+  retentionDays: number;
+  lastRun: BackupRun;
+  /** Carried across failed runs so freshness checks see the real gap. */
+  lastSuccess: { backupId: string; finishedAt: string; sizes: BackupSizes } | null;
+}
+
+/** Reads the previous status; a missing or torn file is simply "no status". */
+export function readStatus(backupsDir: string): BackupStatus | null {
+  const path = join(backupsDir, STATUS_FILE);
+  if (!existsSync(path)) return null;
+  try {
+    return JSON.parse(readFileSync(path, 'utf8')) as BackupStatus;
+  } catch {
+    return null;
+  }
+}
+
+/** Atomic write (staging + rename) so readers never see a torn file. */
+export async function writeStatus(backupsDir: string, status: BackupStatus): Promise<void> {
+  const path = join(backupsDir, STATUS_FILE);
+  const staging = `${path}.tmp-${process.pid}`;
+  await writeFile(staging, JSON.stringify(status, null, 2) + '\n');
+  await rename(staging, path);
+}
diff --git a/apps/backup/tsconfig.json b/apps/backup/tsconfig.json
new file mode 100644
index 0000000..1e20cd9
--- /dev/null
+++ b/apps/backup/tsconfig.json
@@ -0,0 +1,7 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "outDir": "dist"
+  },
+  "include": ["src"]
+}
diff --git a/apps/backup/vitest.config.ts b/apps/backup/vitest.config.ts
new file mode 100644
index 0000000..4ac6027
--- /dev/null
+++ b/apps/backup/vitest.config.ts
@@ -0,0 +1,7 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'node',
+  },
+});
diff --git a/deploy/backup/restore.sh b/deploy/backup/restore.sh
new file mode 100755
index 0000000..3baa1f9
--- /dev/null
+++ b/deploy/backup/restore.sh
@@ -0,0 +1,40 @@
+#!/usr/bin/env sh
+# Restore a nightly backup set (ADR 0015, operations.md §Backup & restore).
+#
+# Run from the stage directory (where docker-compose.yml and .env live):
+#   deploy/backup/restore.sh <backup-id>          # e.g. 20260711-030000
+#
+# Steps (the documented runbook, automated): stop the app services (the db
+# stays up — pg_restore needs it), replay the dump and the uploads/plugins
+# archive through the backup sidecar image, start the stack, check /readyz.
+set -eu
+
+if [ $# -ne 1 ]; then
+  echo "usage: $0 <backup-id>   (list sets: docker compose exec backup ls /backups)" >&2
+  exit 2
+fi
+backup_id="$1"
+
+echo "Stopping app services (db keeps running) ..."
+docker compose stop web api collab
+
+echo "Restoring set ${backup_id} ..."
+docker compose run --rm --no-deps backup node dist/restore.js "${backup_id}"
+
+echo "Starting the stack ..."
+docker compose up -d
+
+api_port="$(docker compose port api 3000 2>/dev/null | head -n1)"
+if [ -n "${api_port}" ]; then
+  echo "Waiting for readiness on ${api_port} ..."
+  for _ in $(seq 1 30); do
+    if curl -sf "http://${api_port}/api/v1/readyz" >/dev/null 2>&1; then
+      echo "Restore complete — /readyz is green. Spot-check a page and a file."
+      exit 0
+    fi
+    sleep 2
+  done
+  echo "Restore applied, but /readyz did not turn green within 60 s — check 'docker compose logs api'." >&2
+  exit 1
+fi
+echo "Restore applied. Verify /readyz through your reverse proxy."
diff --git a/deploy/compose/.env.example b/deploy/compose/.env.example
index 7b4a9ed..7a6177e 100644
--- a/deploy/compose/.env.example
+++ b/deploy/compose/.env.example
@@ -48,6 +48,23 @@ SMTP_USER=wiki@example.com
 SMTP_PASS=change-me
 SMTP_FROM=Dorfteich <wiki@example.com>
 
+# --- backups (ADR 0015, issue #83) --------------------------------------------
+# The backup sidecar dumps the database and archives the uploads/plugins
+# volumes nightly onto the `backups` volume; restore via
+# deploy/backup/restore.sh <backup-id>. All values optional.
+# Daily run time HH:MM in TZ (default 03:00; set TZ for stage-local time,
+# e.g. TZ=Europe/Berlin — unset means UTC).
+#TZ=Europe/Berlin
+#BACKUP_TIME=03:00
+# Local retention in days: 30 (default) for Prod, 7 for Test/Int (ADR 0015).
+#BACKUP_RETENTION_DAYS=30
+# Failure alert: recipient (unset = no mail, failures only in the logs and
+# status.json), mail language (de|en), and the label used in the subject
+# (defaults to the compose project name).
+#BACKUP_MAIL_TO=ops@example.com
+#BACKUP_MAIL_LOCALE=en
+#BACKUP_INSTANCE_LABEL=dorfteich-test
+
 # --- first-run setup (optional pre-seeding, issue #80) ------------------------
 # A fresh (empty) database makes the instance require the browser setup
 # wizard. Automated deploys can skip it entirely by pre-seeding the Site
diff --git a/deploy/compose/docker-compose.yml b/deploy/compose/docker-compose.yml
index 8a34757..8773f91 100644
--- a/deploy/compose/docker-compose.yml
+++ b/deploy/compose/docker-compose.yml
@@ -127,6 +127,52 @@ services:
         condition: service_healthy
     <<: *logging
 
+  # Backup sidecar (ADR 0015, issue #83): nightly `pg_dump -Fc` + one tar of
+  # the uploads/plugins volumes as a consistent restore set on the `backups`
+  # volume, prune by retention, `status.json`, failure mail directly via SMTP
+  # (the api may be the broken part). Restore runs through
+  # deploy/backup/restore.sh, which drives this same image.
+  backup:
+    image: ${IMAGE_PREFIX:-dorfteich}-backup:${TAG:-latest}
+    build:
+      context: ../..
+      dockerfile: apps/backup/Dockerfile
+      args:
+        APP_VERSION: ${TAG:-latest}
+    restart: unless-stopped
+    environment:
+      NODE_ENV: production
+      LOG_LEVEL: ${LOG_LEVEL:-info}
+      DATABASE_URL: postgresql://dorfteich:${POSTGRES_PASSWORD:?set in .env}@db:5432/dorfteich
+      # Daily run time (HH:MM) in TZ; retention 30 d default, 7 d on Test/Int.
+      TZ: ${TZ:-}
+      BACKUP_TIME: ${BACKUP_TIME:-}
+      BACKUP_RETENTION_DAYS: ${BACKUP_RETENTION_DAYS:-}
+      # Failure-alert recipient; empty disables the mail (logged instead).
+      BACKUP_MAIL_TO: ${BACKUP_MAIL_TO:-}
+      BACKUP_MAIL_LOCALE: ${BACKUP_MAIL_LOCALE:-}
+      BACKUP_INSTANCE_LABEL: ${BACKUP_INSTANCE_LABEL:-${COMPOSE_PROJECT_NAME:-dorfteich}}
+      # Same SMTP resolution as the api: explicit env wins, the wizard-written
+      # secret store fills the gaps (issue #80).
+      SMTP_HOST: ${SMTP_HOST:-}
+      SMTP_PORT: ${SMTP_PORT:-}
+      SMTP_SECURE: ${SMTP_SECURE:-}
+      SMTP_USER: ${SMTP_USER:-}
+      SMTP_PASS: ${SMTP_PASS:-}
+      SMTP_FROM: ${SMTP_FROM:-}
+    networks: [internal]
+    volumes:
+      # Write access to uploads/plugins is for the restore path only; the
+      # nightly run just reads them into the archive.
+      - uploads:/data/uploads
+      - plugins:/data/plugins
+      - secrets:/data/secrets:ro
+      - backups:/backups
+    depends_on:
+      db:
+        condition: service_healthy
+    <<: *logging
+
   db:
     image: postgres:17.5-alpine
     restart: unless-stopped
@@ -182,3 +228,4 @@ volumes:
   uploads:
   plugins:
   secrets:
+  backups:
diff --git a/docs/architecture/deployment.md b/docs/architecture/deployment.md
index 3c9fbca..a6e0b40 100644
--- a/docs/architecture/deployment.md
+++ b/docs/architecture/deployment.md
@@ -23,7 +23,9 @@ Every stage (and every self-hosted instance) runs the same services:
 | `gotenberg` | `gotenberg/gotenberg:<pinned>`       | internal only                                                   |
 | `backup`    | `dorfteich-backup`                   | cron sidecar: pg_dump, volume archive, prune, mirror (ADR 0015) |
 
-Volumes: `db-data`, `uploads` (uploads + installed plugins), `backups`.
+Volumes: `db-data`, `uploads`, `plugins` (installed plugin bundles),
+`secrets` (wizard-written secret store), `backups` (restore sets +
+`status.json`).
 Networks: `frontend` (reverse proxy ↔ web/api/collab) and `internal`
 (api/collab ↔ db/pandoc/gotenberg); db and converters are never exposed.
 
diff --git a/docs/architecture/operations.md b/docs/architecture/operations.md
index 1218698..039dbba 100644
--- a/docs/architecture/operations.md
+++ b/docs/architecture/operations.md
@@ -36,14 +36,19 @@ monitoring, structured logs, backup alerting — no dedicated metrics stack.
 
 ## Backup & restore (operational view of ADR 0015)
 
-- Nightly at 03:00 stage-local time: `pg_dump -Fc` → uploads/plugins volume
-  archive → prune (> 30 days Prod, > 7 days Test/Int) → rsync mirror to
-  BASEL `/home/RAID/BACKUPS/dorfteich-prod/` (Prod only, dedicated
-  `dorfteich-backup` user).
-- **Restore runbook** (also the Prod-relocation procedure):
-  1. `docker compose down` (keep volumes),
+- Nightly at 03:00 stage-local time (sidecar `backup`, issue #83; env
+  `BACKUP_TIME`/`TZ`): `pg_dump -Fc` → uploads/plugins volume archive (one
+  tar, same backup id `YYYYMMDD-HHMMSS`) → prune (`BACKUP_RETENTION_DAYS`,
+  30 default / 7 Test+Int; the newest complete set always survives) →
+  `status.json` on the `backups` volume → on failure a mail directly via
+  the instance SMTP to `BACKUP_MAIL_TO`. Mirror to BASEL is issue #84.
+- **On-demand run**: `docker compose run --rm -e BACKUP_RUN_ONCE=1 backup`
+  (exit code = outcome); list sets with `docker compose exec backup ls /backups`.
+- **Restore runbook** (also the Prod-relocation procedure) — automated by
+  `deploy/backup/restore.sh <backup-id>`, run from the stage directory:
+  1. stop the app services (`web`, `api`, `collab`; the db stays up),
   2. restore DB: `pg_restore --clean --if-exists` into the `db` container,
-  3. restore volume: unpack the matching uploads archive,
+  3. restore volume: unpack the matching uploads/plugins archive,
   4. `docker compose up -d`, verify `/readyz`, spot-check a page + a file.
 - **Drills**: monthly automated restore of the latest Prod dump into a
   scratch database on Test with a row-count sanity report; quarterly manual
diff --git a/packages/shared/i18n/de/mails.json b/packages/shared/i18n/de/mails.json
index 7a7ffbd..367b41a 100644
--- a/packages/shared/i18n/de/mails.json
+++ b/packages/shared/i18n/de/mails.json
@@ -21,5 +21,14 @@
     "body": "diese Testnachricht bestätigt, dass dein Dorfteich E-Mails über den konfigurierten SMTP-Server versenden kann. Deine Instanz erreichst du hier:",
     "action": "Dorfteich öffnen",
     "expiry": "Du kannst diese E-Mail einfach löschen."
+  },
+  "backupFailed": {
+    "subject": "[{{instance}}] Backup fehlgeschlagen ({{backupId}})",
+    "intro": "Der nächtliche Backup-Lauf auf {{instance}} ist fehlgeschlagen.",
+    "backupId": "Backup-ID: {{backupId}}",
+    "error": "Fehler: {{error}}",
+    "lastSuccess": "Letztes erfolgreiches Backup: {{finishedAt}}",
+    "lastSuccessNever": "Letztes erfolgreiches Backup: noch keines",
+    "hint": "Prüfe die Sidecar-Logs (docker compose logs backup) und die status.json auf dem Backups-Volume."
   }
 }
diff --git a/packages/shared/i18n/en/mails.json b/packages/shared/i18n/en/mails.json
index fa6fdde..cb0c3ad 100644
--- a/packages/shared/i18n/en/mails.json
+++ b/packages/shared/i18n/en/mails.json
@@ -21,5 +21,14 @@
     "body": "this test message confirms that your Dorfteich can send e-mail through the configured SMTP server. You can reach your instance here:",
     "action": "Open Dorfteich",
     "expiry": "You can simply delete this e-mail."
+  },
+  "backupFailed": {
+    "subject": "[{{instance}}] Backup failed ({{backupId}})",
+    "intro": "The nightly backup run on {{instance}} failed.",
+    "backupId": "Backup id: {{backupId}}",
+    "error": "Error: {{error}}",
+    "lastSuccess": "Last successful backup: {{finishedAt}}",
+    "lastSuccessNever": "Last successful backup: none yet",
+    "hint": "Check the sidecar logs (docker compose logs backup) and status.json on the backups volume."
   }
 }
diff --git a/packages/shared/src/env.ts b/packages/shared/src/env.ts
index 84de051..ceb7c80 100644
--- a/packages/shared/src/env.ts
+++ b/packages/shared/src/env.ts
@@ -27,6 +27,25 @@ const databaseUrl = z
  */
 const collabTokenSecret = z.string().min(16).default('dev-insecure-collab-token-secret-change-me');
 
+/**
+ * SMTP delivery fields, shared by the api (transactional mail) and the
+ * backup sidecar (failure alert mail, issue #83). Defaults match the
+ * Mailpit container from the dev overlay; production instances configure
+ * their real relay in the stage `.env` or through the M8 setup wizard,
+ * whose secret store both services overlay the same way.
+ */
+const smtpFields = {
+  SMTP_HOST: z.string().default('localhost'),
+  SMTP_PORT: z.coerce.number().int().default(1025),
+  SMTP_SECURE: z
+    .enum(['true', 'false'])
+    .default('false')
+    .transform((value) => value === 'true'),
+  SMTP_USER: z.string().optional(),
+  SMTP_PASS: z.string().optional(),
+  SMTP_FROM: z.string().default('Dorfteich <no-reply@localhost>'),
+};
+
 export const apiEnvSchema = z.object({
   NODE_ENV: nodeEnv,
   PORT: z.coerce.number().int().min(1).max(65535).default(3000),
@@ -41,20 +60,7 @@ export const apiEnvSchema = z.object({
     .transform((value) => value === 'true'),
   /** Public base URL of this instance — used in e-mail links. */
   APP_BASE_URL: z.string().url().default('http://localhost:5173'),
-  /**
-   * SMTP delivery. Defaults match the Mailpit container from the dev
-   * overlay; production instances configure their real relay here (the
-   * M8 setup wizard writes these).
-   */
-  SMTP_HOST: z.string().default('localhost'),
-  SMTP_PORT: z.coerce.number().int().default(1025),
-  SMTP_SECURE: z
-    .enum(['true', 'false'])
-    .default('false')
-    .transform((value) => value === 'true'),
-  SMTP_USER: z.string().optional(),
-  SMTP_PASS: z.string().optional(),
-  SMTP_FROM: z.string().default('Dorfteich <no-reply@localhost>'),
+  ...smtpFields,
   /**
    * Filesystem root for uploaded files (ADR 0011). The compose stack
    * mounts the `uploads` volume at `/data/uploads` and sets this
@@ -132,6 +138,41 @@ export const collabEnvSchema = z.object({
 
 export type CollabEnv = z.infer<typeof collabEnvSchema>;
 
+/**
+ * Configuration for the backup sidecar (ADR 0015, issue #83). It talks to
+ * the same database and mounts the same data volumes as the api, plus its
+ * own `backups` volume for the nightly restore sets and `status.json`.
+ */
+export const backupEnvSchema = z.object({
+  NODE_ENV: nodeEnv,
+  LOG_LEVEL: logLevel,
+  APP_VERSION: appVersion,
+  DATABASE_URL: databaseUrl,
+  /** Where restore sets and `status.json` are written (the `backups` volume). */
+  BACKUPS_DIR: z.string().min(1).default('./data/backups'),
+  /** Same mounts as the api — archived together as one restore set. */
+  UPLOADS_DIR: z.string().min(1).default('./data/uploads'),
+  PLUGINS_DIR: z.string().min(1).default('./data/plugins'),
+  /** Daily run time as HH:MM, interpreted in the container's TZ. */
+  BACKUP_TIME: z
+    .string()
+    .regex(/^([01]\d|2[0-3]):[0-5]\d$/, 'must be HH:MM (24h)')
+    .default('03:00'),
+  /** Local retention in days: 30 for Prod, 7 for Test/Int (ADR 0015). */
+  BACKUP_RETENTION_DAYS: z.coerce.number().int().min(1).default(30),
+  /** Failure-alert recipient; unset disables the mail (logged instead). */
+  BACKUP_MAIL_TO: z.string().optional(),
+  /** Language of the failure mail (ADR 0012 — both exist, operator picks). */
+  BACKUP_MAIL_LOCALE: z.enum(['de', 'en']).default('en'),
+  /** Instance label in the mail subject, e.g. "dorfteich-test". */
+  BACKUP_INSTANCE_LABEL: z.string().optional(),
+  ...smtpFields,
+  /** Read-only view of the wizard-written secret store (issue #80). */
+  SECRETS_FILE: z.string().min(1).default('./data/secrets.env'),
+});
+
+export type BackupEnv = z.infer<typeof backupEnvSchema>;
+
 export function parseEnv<Schema extends z.ZodTypeAny>(
   schema: Schema,
   env: Record<string, string | undefined>,
diff --git a/packages/shared/src/index.ts b/packages/shared/src/index.ts
index fe5427c..f8895a6 100644
--- a/packages/shared/src/index.ts
+++ b/packages/shared/src/index.ts
@@ -17,6 +17,7 @@ export * from './pages';
 export * from './permissions';
 export * from './plugins';
 export * from './search';
+export * from './secret-store';
 export * from './setup';
 export * from './ponds';
 export * from './quotas';
diff --git a/packages/shared/src/secret-store.ts b/packages/shared/src/secret-store.ts
new file mode 100644
index 0000000..db67e31
--- /dev/null
+++ b/packages/shared/src/secret-store.ts
@@ -0,0 +1,62 @@
+/**
+ * The env-backed secret store (security.md §Secrets, issue #80): secrets the
+ * setup wizard collects in the browser (SMTP credentials) are persisted as a
+ * mode-600 dotenv-style file on a volume — never as database rows. The file
+ * extends the environment: `overlayEnv` fills only variables the process
+ * environment does not set. These pure format/merge helpers live in shared
+ * because two services read the store the api writes: the api itself and the
+ * backup sidecar (issue #83), which needs the wizard's SMTP relay for its
+ * failure mail. File I/O stays with each service.
+ */
+
+/** Parses the dotenv-style store content. Ignores blank lines and comments. */
+export function parseSecretsFile(content: string): Record<string, string> {
+  const secrets: Record<string, string> = {};
+  for (const line of content.split('\n')) {
+    const trimmed = line.trim();
+    if (!trimmed || trimmed.startsWith('#')) continue;
+    const eq = trimmed.indexOf('=');
+    if (eq <= 0) continue;
+    const key = trimmed.slice(0, eq).trim();
+    let value = trimmed.slice(eq + 1).trim();
+    if (value.startsWith('"') && value.endsWith('"') && value.length >= 2) {
+      value = value.slice(1, -1).replace(/\\n/g, '\n').replace(/\\"/g, '"').replace(/\\\\/g, '\\');
+    }
+    secrets[key] = value;
+  }
+  return secrets;
+}
+
+/** Serializes secrets with double-quoted, escaped values (dotenv-compatible). */
+export function serializeSecrets(secrets: Record<string, string>): string {
+  const lines = [
+    '# Managed by Dorfteich (setup wizard). Values here fill environment',
+    '# variables that the container environment does not set explicitly.',
+  ];
+  for (const [key, value] of Object.entries(secrets)) {
+    const escaped = value.replace(/\\/g, '\\\\').replace(/"/g, '\\"').replace(/\n/g, '\\n');
+    lines.push(`${key}="${escaped}"`);
+  }
+  return lines.join('\n') + '\n';
+}
+
+/**
+ * Merges the store under the real environment: explicit env vars win, store
+ * values fill the gaps (and Zod defaults fill whatever remains at parse
+ * time). Empty strings count as unset on both sides — compose passes
+ * `${SMTP_HOST:-}` as `""` for variables the stage `.env` does not define,
+ * and those must not shadow wizard-written store values or schema defaults.
+ */
+export function overlayEnv(
+  env: Record<string, string | undefined>,
+  secrets: Record<string, string>,
+): Record<string, string | undefined> {
+  const merged: Record<string, string | undefined> = {};
+  for (const [key, value] of Object.entries(secrets)) {
+    if (value !== '') merged[key] = value;
+  }
+  for (const [key, value] of Object.entries(env)) {
+    if (value !== undefined && value !== '') merged[key] = value;
+  }
+  return merged;
+}
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 3a17c8b..226cdf6 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -166,6 +166,34 @@ importers:
         specifier: ^3.0.0
         version: 3.2.6(@types/node@26.1.0)(jiti@2.7.0)(jsdom@26.1.0)(terser@5.48.0)(tsx@4.23.0)
 
+  apps/backup:
+    dependencies:
+      '@dorfteich/shared':
+        specifier: workspace:*
+        version: link:../../packages/shared
+      nodemailer:
+        specifier: ^9.0.3
+        version: 9.0.3
+      pino:
+        specifier: ^9.6.0
+        version: 9.14.0
+      zod:
+        specifier: ^3.25.76
+        version: 3.25.76
+    devDependencies:
+      '@types/node':
+        specifier: ^26.1.0
+        version: 26.1.0
+      '@types/nodemailer':
+        specifier: ^8.0.1
+        version: 8.0.1
+      tsx:
+        specifier: ^4.19.0
+        version: 4.23.0
+      vitest:
+        specifier: ^3.0.0
+        version: 3.2.6(@types/node@26.1.0)(jiti@2.7.0)(jsdom@26.1.0)(terser@5.48.0)(tsx@4.23.0)
+
   apps/collab:
     dependencies:
       '@dorfteich/shared':