diff --git a/.gitea/workflows/cd.yml b/.gitea/workflows/cd.yml
index 04fb242..edd7fc1 100644
--- a/.gitea/workflows/cd.yml
+++ b/.gitea/workflows/cd.yml
@@ -40,6 +40,13 @@ jobs:
           docker push $IMAGE_BASE-api:${{ github.sha }}
           docker push $IMAGE_BASE-api:test
 
+      - name: Build and push collab image
+        run: |
+          docker build -f apps/collab/Dockerfile --build-arg APP_VERSION=${{ github.sha }} \
+            -t $IMAGE_BASE-collab:${{ github.sha }} -t $IMAGE_BASE-collab:test .
+          docker push $IMAGE_BASE-collab:${{ github.sha }}
+          docker push $IMAGE_BASE-collab:test
+
   deploy-test:
     name: Deploy to Test
     needs: build-push
@@ -107,6 +114,7 @@ jobs:
         run: |
           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 }}
 
       - name: Set up SSH
         run: |
diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index 8cfb697..44f398d 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -157,3 +157,6 @@ jobs:
 
       - name: Build api image
         run: docker build -f apps/api/Dockerfile --build-arg APP_VERSION=${{ github.sha }} -t dorfteich-api:ci .
+
+      - name: Build collab image
+        run: docker build -f apps/collab/Dockerfile --build-arg APP_VERSION=${{ github.sha }} -t dorfteich-collab:ci .
diff --git a/apps/collab/Dockerfile b/apps/collab/Dockerfile
new file mode 100644
index 0000000..84285fe
--- /dev/null
+++ b/apps/collab/Dockerfile
@@ -0,0 +1,26 @@
+# Build context is the repository root (workspace build):
+#   docker build -f apps/collab/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/collab ./apps/collab
+RUN pnpm install --frozen-lockfile --filter @dorfteich/collab... \
+  && pnpm --filter @dorfteich/shared build \
+  && pnpm --filter @dorfteich/collab build \
+  # Self-contained production bundle (prod deps only) at /out.
+  && pnpm --filter @dorfteich/collab deploy --prod --legacy /out \
+  && cp -r apps/collab/dist /out/dist
+
+FROM node:22.15-alpine
+ARG APP_VERSION=0.0.0-dev
+ENV NODE_ENV=production APP_VERSION=${APP_VERSION}
+WORKDIR /app
+COPY --from=build --chown=node:node /out /app
+USER node
+EXPOSE 3000
+HEALTHCHECK --interval=30s --timeout=5s --retries=3 \
+  CMD node -e "fetch('http://127.0.0.1:'+(process.env.PORT||3000)+'/healthz').then(r=>process.exit(r.ok?0:1)).catch(()=>process.exit(1))"
+CMD ["node", "dist/index.js"]
diff --git a/apps/collab/package.json b/apps/collab/package.json
index d34459c..e06c7aa 100644
--- a/apps/collab/package.json
+++ b/apps/collab/package.json
@@ -2,14 +2,27 @@
   "name": "@dorfteich/collab",
   "version": "0.0.0",
   "private": true,
+  "type": "module",
   "description": "Dorfteich collaboration server (Yjs/Hocuspocus)",
   "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:*",
+    "@hocuspocus/server": "^4.3.0",
+    "pg": "^8.16.0",
+    "pino": "^9.6.0"
+  },
   "devDependencies": {
+    "@types/node": "^26.1.0",
+    "@types/pg": "^8.11.0",
+    "tsx": "^4.19.0",
     "vitest": "^3.0.0"
   }
 }
diff --git a/apps/collab/src/db.ts b/apps/collab/src/db.ts
new file mode 100644
index 0000000..5e2e51a
--- /dev/null
+++ b/apps/collab/src/db.ts
@@ -0,0 +1,34 @@
+import { Pool } from 'pg';
+
+import type { DatabaseProbe } from './health.js';
+
+/**
+ * A small connection pool to the same PostgreSQL the api uses (ADR 0002). The
+ * collab skeleton (issue #33) only needs it for the health probe; the document
+ * persistence hooks (issue #35) will reuse this pool for load/store.
+ */
+export function createPool(connectionString: string): Pool {
+  return new Pool({
+    connectionString,
+    max: 4,
+    // Fail fast on health probes instead of hanging when the db is unreachable.
+    connectionTimeoutMillis: 5000,
+    query_timeout: 5000,
+  });
+}
+
+/** Probe database reachability for the health endpoint. Never throws. */
+export async function pingDatabase(pool: Pool): Promise<DatabaseProbe> {
+  try {
+    await pool.query('SELECT 1');
+    return { ok: true };
+  } catch (error) {
+    return { ok: false, detail: shortMessage(error) };
+  }
+}
+
+function shortMessage(error: unknown): string {
+  const message = error instanceof Error ? error.message : String(error);
+  // Keep the health output single-line and free of any connection string.
+  return message.split('\n').filter(Boolean).slice(-1)[0]?.slice(0, 200) ?? 'unknown error';
+}
diff --git a/apps/collab/src/health.test.ts b/apps/collab/src/health.test.ts
new file mode 100644
index 0000000..d63bd80
--- /dev/null
+++ b/apps/collab/src/health.test.ts
@@ -0,0 +1,44 @@
+import { describe, expect, it } from 'vitest';
+
+import { buildHealthReport, isHealthRequest } from './health.js';
+
+describe('buildHealthReport', () => {
+  it('reports ok with HTTP 200 when the database is reachable', () => {
+    const { httpStatus, body } = buildHealthReport('1.2.3', { ok: true });
+    expect(httpStatus).toBe(200);
+    expect(body.status).toBe('ok');
+    expect(body.service).toBe('collab');
+    expect(body.version).toBe('1.2.3');
+    expect(body.checks).toEqual([{ name: 'database', status: 'ok' }]);
+    expect(typeof body.time).toBe('string');
+  });
+
+  it('reports unhealthy with HTTP 503 and the detail when the database is down', () => {
+    const { httpStatus, body } = buildHealthReport('1.2.3', {
+      ok: false,
+      detail: 'connection refused',
+    });
+    expect(httpStatus).toBe(503);
+    expect(body.status).toBe('unhealthy');
+    expect(body.checks).toEqual([
+      { name: 'database', status: 'failed', detail: 'connection refused' },
+    ]);
+  });
+});
+
+describe('isHealthRequest', () => {
+  it('matches the internal and proxied health paths on GET/HEAD', () => {
+    expect(isHealthRequest('GET', '/healthz')).toBe(true);
+    expect(isHealthRequest('HEAD', '/healthz')).toBe(true);
+    expect(isHealthRequest('GET', '/healthz?probe=1')).toBe(true);
+    expect(isHealthRequest('GET', '/collab/healthz')).toBe(true);
+    expect(isHealthRequest('GET', '/collab/healthz/')).toBe(true);
+  });
+
+  it('does not match other methods, the WebSocket path, or the root', () => {
+    expect(isHealthRequest('POST', '/healthz')).toBe(false);
+    expect(isHealthRequest('GET', '/collab')).toBe(false);
+    expect(isHealthRequest('GET', '/')).toBe(false);
+    expect(isHealthRequest('GET', undefined)).toBe(false);
+  });
+});
diff --git a/apps/collab/src/health.ts b/apps/collab/src/health.ts
new file mode 100644
index 0000000..4f4b096
--- /dev/null
+++ b/apps/collab/src/health.ts
@@ -0,0 +1,64 @@
+import type { HealthResponse } from '@dorfteich/shared';
+
+/** Result of a single dependency probe. */
+export interface DatabaseProbe {
+  ok: boolean;
+  detail?: string;
+}
+
+/**
+ * Liveness + dependency report for the collab service. Unlike the api, which
+ * splits pure liveness (`/healthz`) from readiness (`/readyz`), the collab
+ * skeleton only needs the one endpoint the compose healthcheck and pipeline
+ * smoke test hit, so `/healthz` also carries the database probe (issue #33).
+ */
+export interface CollabHealthReport {
+  status: 'ok' | 'unhealthy';
+  service: HealthResponse['service'];
+  version: string;
+  time: string;
+  checks: { name: 'database'; status: 'ok' | 'failed'; detail?: string }[];
+}
+
+/** Build the health payload and the HTTP status a monitor should see. */
+export function buildHealthReport(
+  version: string,
+  database: DatabaseProbe,
+): { httpStatus: 200 | 503; body: CollabHealthReport } {
+  const body: CollabHealthReport = {
+    status: database.ok ? 'ok' : 'unhealthy',
+    service: 'collab',
+    version,
+    time: new Date().toISOString(),
+    checks: [
+      {
+        name: 'database',
+        status: database.ok ? 'ok' : 'failed',
+        ...(database.detail ? { detail: database.detail } : {}),
+      },
+    ],
+  };
+  return { httpStatus: database.ok ? 200 : 503, body };
+}
+
+/**
+ * Whether a plain HTTP request is a health probe. Matches both the container-
+ * internal path (`/healthz`, used by the Docker healthcheck) and the path seen
+ * through the host reverse proxy, which routes `/collab*` to this service
+ * without stripping the prefix (deployment.md), i.e. `/collab/healthz`.
+ */
+export function isHealthRequest(method: string | undefined, url: string | undefined): boolean {
+  if (method !== undefined && method !== 'GET' && method !== 'HEAD') {
+    return false;
+  }
+  const pathname = pathnameOf(url);
+  return pathname === '/healthz' || pathname === '/collab/healthz';
+}
+
+function pathnameOf(url: string | undefined): string {
+  if (!url) {
+    return '';
+  }
+  // The base only matters to parse a path-only request-target; it is discarded.
+  return new URL(url, 'http://collab.internal').pathname.replace(/\/+$/, '') || '/';
+}
diff --git a/apps/collab/src/index.ts b/apps/collab/src/index.ts
index 9d6de4d..24756bf 100644
--- a/apps/collab/src/index.ts
+++ b/apps/collab/src/index.ts
@@ -1,3 +1,35 @@
-// Placeholder entry point; the Hocuspocus server arrives with milestone M3
-// (issue #33). The package exists so workspace tooling covers it from day one.
-export const SERVICE_NAME = 'collab';
+import { collabEnvSchema, parseEnv } from '@dorfteich/shared';
+
+import { createPool, pingDatabase } from './db.js';
+import { createLogger } from './logger.js';
+import { createCollabServer } from './server.js';
+
+/**
+ * Entry point of the collaboration server (ADR 0003). Validates the
+ * environment, opens a database pool for the health probe, and starts the
+ * Hocuspocus server. A clean shutdown closes both so the container stops fast.
+ */
+async function bootstrap(): Promise<void> {
+  // Validate configuration up front; crash with a readable list on problems.
+  const env = parseEnv(collabEnvSchema, process.env);
+  const logger = createLogger(env);
+  const pool = createPool(env.DATABASE_URL);
+
+  const server = createCollabServer({
+    version: env.APP_VERSION,
+    logger,
+    pingDatabase: () => pingDatabase(pool),
+  });
+
+  await server.listen(env.PORT);
+  logger.info({ event: 'listen', port: env.PORT }, 'collaboration server listening');
+
+  const shutdown = (signal: NodeJS.Signals): void => {
+    logger.info({ event: 'shutdown', signal }, 'shutting down');
+    void Promise.allSettled([server.destroy(), pool.end()]).then(() => process.exit(0));
+  };
+  process.on('SIGTERM', () => shutdown('SIGTERM'));
+  process.on('SIGINT', () => shutdown('SIGINT'));
+}
+
+void bootstrap();
diff --git a/apps/collab/src/logger.ts b/apps/collab/src/logger.ts
new file mode 100644
index 0000000..74601b1
--- /dev/null
+++ b/apps/collab/src/logger.ts
@@ -0,0 +1,15 @@
+import { pino, type Logger } from 'pino';
+
+import type { CollabEnv } from '@dorfteich/shared';
+
+/**
+ * A pino JSON logger, matching the api's structured output (ADR 0003 asks the
+ * collab service to log consistently with the api). Every line carries
+ * `service: "collab"` so logs from the two containers are easy to tell apart.
+ */
+export function createLogger(env: Pick<CollabEnv, 'LOG_LEVEL' | 'APP_VERSION'>): Logger {
+  return pino({
+    level: env.LOG_LEVEL,
+    base: { service: 'collab', version: env.APP_VERSION },
+  });
+}
diff --git a/apps/collab/src/server.test.ts b/apps/collab/src/server.test.ts
new file mode 100644
index 0000000..f8c8af5
--- /dev/null
+++ b/apps/collab/src/server.test.ts
@@ -0,0 +1,65 @@
+import { pino } from 'pino';
+import { afterAll, beforeAll, describe, expect, it, vi } from 'vitest';
+
+import { createCollabServer } from './server.js';
+import type { CollabHealthReport, DatabaseProbe } from './health.js';
+
+// A silent logger; these tests assert behaviour, not log output.
+const logger = pino({ enabled: false });
+
+describe('collab server', () => {
+  let server: ReturnType<typeof createCollabServer>;
+  let baseUrl: string;
+  let wsUrl: string;
+  const probe = vi.fn<() => Promise<DatabaseProbe>>();
+
+  beforeAll(async () => {
+    server = createCollabServer({ version: 'test-version', logger, pingDatabase: probe });
+    // Port 0 binds an ephemeral free port.
+    await server.listen(0);
+    const { port } = server.address;
+    baseUrl = `http://127.0.0.1:${port}`;
+    wsUrl = `ws://127.0.0.1:${port}`;
+  });
+
+  afterAll(async () => {
+    await server.destroy();
+  });
+
+  it('serves /healthz as 200 with the report when the database is reachable', async () => {
+    probe.mockResolvedValueOnce({ ok: true });
+    const res = await fetch(`${baseUrl}/healthz`);
+    expect(res.status).toBe(200);
+    expect(res.headers.get('content-type')).toContain('application/json');
+    const body = (await res.json()) as CollabHealthReport;
+    expect(body).toMatchObject({ status: 'ok', service: 'collab', version: 'test-version' });
+    expect(body.checks).toEqual([{ name: 'database', status: 'ok' }]);
+  });
+
+  it('serves /healthz as 503 when the database probe fails', async () => {
+    probe.mockResolvedValueOnce({ ok: false, detail: 'boom' });
+    const res = await fetch(`${baseUrl}/healthz`);
+    expect(res.status).toBe(503);
+    const body = (await res.json()) as CollabHealthReport;
+    expect(body.status).toBe('unhealthy');
+  });
+
+  it('leaves non-health HTTP requests to Hocuspocus without probing the db', async () => {
+    const callsBefore = probe.mock.calls.length;
+    const res = await fetch(`${baseUrl}/`);
+    expect(res.status).toBe(200);
+    expect(await res.text()).toContain('Hocuspocus');
+    expect(probe.mock.calls.length).toBe(callsBefore);
+  });
+
+  it('accepts a WebSocket handshake on the collab path', async () => {
+    const socket = new WebSocket(`${wsUrl}/collab`);
+    await expect(
+      new Promise<void>((resolve, reject) => {
+        socket.addEventListener('open', () => resolve());
+        socket.addEventListener('error', () => reject(new Error('handshake failed')));
+      }),
+    ).resolves.toBeUndefined();
+    socket.close();
+  });
+});
diff --git a/apps/collab/src/server.ts b/apps/collab/src/server.ts
new file mode 100644
index 0000000..7de6cea
--- /dev/null
+++ b/apps/collab/src/server.ts
@@ -0,0 +1,65 @@
+import { Server } from '@hocuspocus/server';
+import type { Logger } from 'pino';
+
+import { buildHealthReport, isHealthRequest, type DatabaseProbe } from './health.js';
+
+export interface CollabServerDeps {
+  /** Version string surfaced in health responses (image build arg). */
+  version: string;
+  logger: Logger;
+  /** Probe used by the `/healthz` endpoint. Injected so it is easy to test. */
+  pingDatabase: () => Promise<DatabaseProbe>;
+}
+
+/**
+ * The Hocuspocus collaboration server (ADR 0003). This skeleton (issue #33)
+ * wires structured connection logging and a `/healthz` endpoint; it does not
+ * yet authenticate connections (issue #34) or persist documents (issue #35),
+ * so any WebSocket handshake is currently accepted.
+ */
+export function createCollabServer(deps: CollabServerDeps): Server {
+  const { version, logger, pingDatabase } = deps;
+
+  return new Server({
+    name: 'dorfteich-collab',
+    // Suppress Hocuspocus' ASCII start screen; we emit our own pino logs.
+    quiet: true,
+
+    async onConnect({ documentName, socketId }) {
+      logger.info(
+        { event: 'connection.open', documentName, socketId },
+        'collaboration connection opened',
+      );
+    },
+
+    async onDisconnect({ documentName, socketId, clientsCount }) {
+      logger.info(
+        { event: 'connection.close', documentName, socketId, clientsCount },
+        'collaboration connection closed',
+      );
+    },
+
+    async onRequest({ request, response }) {
+      if (!isHealthRequest(request.method, request.url)) {
+        // Not a health probe: let Hocuspocus handle the request.
+        return;
+      }
+
+      const database = await pingDatabase();
+      const { httpStatus, body } = buildHealthReport(version, database);
+      if (!database.ok) {
+        logger.warn(
+          { event: 'health.failed', detail: database.detail },
+          'health check failed: database unreachable',
+        );
+      }
+
+      response.writeHead(httpStatus, { 'Content-Type': 'application/json' });
+      response.end(JSON.stringify(body));
+
+      // Hocuspocus sends a default 200 unless the onRequest chain rejects with
+      // a falsy reason; the response is already written, so stop the chain.
+      return Promise.reject();
+    },
+  });
+}
diff --git a/apps/collab/tsconfig.json b/apps/collab/tsconfig.json
index b0a129d..1e20cd9 100644
--- a/apps/collab/tsconfig.json
+++ b/apps/collab/tsconfig.json
@@ -1,8 +1,6 @@
 {
   "extends": "../../tsconfig.base.json",
   "compilerOptions": {
-    "module": "CommonJS",
-    "moduleResolution": "Node",
     "outDir": "dist"
   },
   "include": ["src"]
diff --git a/apps/web/e2e/smoke.spec.ts b/apps/web/e2e/smoke.spec.ts
index 12571a7..311417f 100644
--- a/apps/web/e2e/smoke.spec.ts
+++ b/apps/web/e2e/smoke.spec.ts
@@ -23,3 +23,13 @@ test('api is live and ready', async ({ request }) => {
   const readyz = await request.get('/api/v1/readyz');
   expect(readyz.ok(), `readyz: ${await readyz.text()}`).toBeTruthy();
 });
+
+test('collab service is live and reachable through the proxy', async ({ request }) => {
+  // The reverse proxy routes /collab to the Hocuspocus container (issue #33);
+  // its /healthz also pings the database, so 200 proves the whole path works.
+  const res = await request.get('/collab/healthz');
+  expect(res.ok(), `collab healthz: ${await res.text()}`).toBeTruthy();
+  const body = await res.json();
+  expect(body.status).toBe('ok');
+  expect(body.service).toBe('collab');
+});
diff --git a/deploy/compose/.env.example b/deploy/compose/.env.example
index baa595e..535b5ec 100644
--- a/deploy/compose/.env.example
+++ b/deploy/compose/.env.example
@@ -13,10 +13,12 @@ IMAGE_PREFIX=dorfteich
 TAG=latest
 
 # --- ports (localhost only; the host reverse proxy routes to these) ---------
-# Suggested per stage on the shared VPS: test 8100/8101, int 8110/8111,
-# prod 8120/8121.
+# Suggested per stage on the shared VPS: test 8100/8101/8102,
+# int 8110/8111/8112, prod 8120/8121/8122 (web/api/collab).
 WEB_PORT=8100
 API_PORT=8101
+# collab (Hocuspocus) WebSocket server; the proxy routes /collab here.
+COLLAB_PORT=8102
 
 # --- behavior ----------------------------------------------------------------
 # pino log level: fatal|error|warn|info|debug|trace
diff --git a/deploy/compose/compose.dev.yml b/deploy/compose/compose.dev.yml
index 5daba9c..520eb5d 100644
--- a/deploy/compose/compose.dev.yml
+++ b/deploy/compose/compose.dev.yml
@@ -50,6 +50,27 @@ services:
       - api-shared-modules:/repo/packages/shared/node_modules
       - pnpm-store:/root/.local/share/pnpm/store
 
+  collab:
+    image: node:22.15-alpine
+    build: !reset null
+    working_dir: /repo
+    command: sh -c "npm i -g pnpm@11 && pnpm install && pnpm --filter @dorfteich/shared build && pnpm --filter @dorfteich/collab start:dev"
+    environment:
+      NODE_ENV: development
+      PORT: '3000'
+      DATABASE_URL: postgresql://dorfteich:${POSTGRES_PASSWORD:-dorfteich}@db:5432/dorfteich
+    ports: !override
+      - '127.0.0.1:3002:3000'
+    volumes:
+      - ../..:/repo
+      - collab-root-modules:/repo/node_modules
+      - collab-app-modules:/repo/apps/collab/node_modules
+      - collab-shared-modules:/repo/packages/shared/node_modules
+      - pnpm-store:/root/.local/share/pnpm/store
+    # No healthcheck block here: the production image's HEALTHCHECK does not
+    # apply to this node:alpine dev image, and nothing depends on it.
+    healthcheck: !reset null
+
   db:
     ports:
       # 5434 on the host to avoid colliding with other local PostgreSQL
@@ -71,4 +92,7 @@ volumes:
   api-root-modules:
   api-app-modules:
   api-shared-modules:
+  collab-root-modules:
+  collab-app-modules:
+  collab-shared-modules:
   pnpm-store:
diff --git a/deploy/compose/docker-compose.yml b/deploy/compose/docker-compose.yml
index e980ced..b8e152f 100644
--- a/deploy/compose/docker-compose.yml
+++ b/deploy/compose/docker-compose.yml
@@ -67,6 +67,34 @@ services:
         condition: service_healthy
     <<: *logging
 
+  collab:
+    image: ${IMAGE_PREFIX:-dorfteich}-collab:${TAG:-latest}
+    build:
+      context: ../..
+      dockerfile: apps/collab/Dockerfile
+      args:
+        APP_VERSION: ${TAG:-latest}
+    restart: unless-stopped
+    environment:
+      NODE_ENV: production
+      PORT: '3000'
+      LOG_LEVEL: ${LOG_LEVEL:-info}
+      DATABASE_URL: postgresql://dorfteich:${POSTGRES_PASSWORD:?set in .env}@db:5432/dorfteich
+    ports:
+      # The host reverse proxy routes /collab here with WebSocket upgrade
+      # (deployment.md, deploy/stages.md).
+      - '127.0.0.1:${COLLAB_PORT:-8102}:3000'
+    networks: [frontend, internal]
+    healthcheck:
+      test: ['CMD-SHELL', 'wget -q -O /dev/null http://127.0.0.1:3000/healthz || exit 1']
+      interval: 30s
+      timeout: 5s
+      retries: 3
+    depends_on:
+      db:
+        condition: service_healthy
+    <<: *logging
+
   db:
     image: postgres:17.5-alpine
     restart: unless-stopped
diff --git a/deploy/stages.md b/deploy/stages.md
index 6310487..ae2a21b 100644
--- a/deploy/stages.md
+++ b/deploy/stages.md
@@ -7,10 +7,10 @@ owner; everything else can be done by CI or a deploy user.
 
 ## Overview
 
-| Stage | Directory                      | Domain                 | Ports (localhost)  |
-| ----- | ------------------------------ | ---------------------- | ------------------ |
-| Test  | `/home/DOCKER/dorfteich-test/` | `test.dorfteich.cloud` | web 8100, api 8101 |
-| Int   | `/home/DOCKER/dorfteich-int/`  | `int.dorfteich.cloud`  | web 8110, api 8111 |
+| Stage | Directory                      | Domain                 | Ports (localhost)               |
+| ----- | ------------------------------ | ---------------------- | ------------------------------- |
+| Test  | `/home/DOCKER/dorfteich-test/` | `test.dorfteich.cloud` | web 8100, api 8101, collab 8102 |
+| Int   | `/home/DOCKER/dorfteich-int/`  | `int.dorfteich.cloud`  | web 8110, api 8111, collab 8112 |
 
 ## 1. Stage directories **[root]**
 
@@ -26,7 +26,10 @@ each stage directory. Set per stage in `.env` (mode 600):
 
 - `POSTGRES_PASSWORD`: unique random value per stage
 - `COMPOSE_PROJECT_NAME`: `dorfteich-test` / `dorfteich-int`
-- `WEB_PORT`/`API_PORT`: 8100/8101 (test), 8110/8111 (int)
+- `WEB_PORT`/`API_PORT`/`COLLAB_PORT`: 8100/8101/8102 (test),
+  8110/8111/8112 (int). **`COLLAB_PORT` must be set per stage** — both
+  stacks share this host, so the compose default (8102) would make the
+  Int collab container collide with Test's; Int needs `COLLAB_PORT=8112`.
 - `IMAGE_PREFIX=gitea.101010.cloud/stwaidele/dorfteich`
 - `TAG`: managed by the CD pipeline (`<git-sha>` on test, `int` on int)
 - `APP_BASE_URL`: `https://test.dorfteich.cloud` / `https://int.dorfteich.cloud`
@@ -62,7 +65,7 @@ test.dorfteich.cloud {
         reverse_proxy 127.0.0.1:8101
     }
     handle /collab* {
-        reverse_proxy 127.0.0.1:8102   # collab service arrives with M3
+        reverse_proxy 127.0.0.1:8102   # Hocuspocus collab (issue #33); Caddy passes the WebSocket upgrade through automatically
     }
     handle {
         reverse_proxy 127.0.0.1:8100
@@ -117,6 +120,7 @@ The pipeline pushes images to the Gitea container registry
 
 - [ ] `https://test.dorfteich.cloud/healthz` → `ok`
 - [ ] `https://test.dorfteich.cloud/api/v1/readyz` → `{"status":"ok",…}`
+- [ ] `https://test.dorfteich.cloud/collab/healthz` → `{"status":"ok","service":"collab",…}`
 - [ ] same for int
 - [ ] runner shows _online_ under Gitea → Settings → Actions → Runners
 - [ ] a test workflow run executes on the runner
diff --git a/docs/architecture/deployment.md b/docs/architecture/deployment.md
index 88d1d65..d29232c 100644
--- a/docs/architecture/deployment.md
+++ b/docs/architecture/deployment.md
@@ -35,6 +35,10 @@ host), routing:
 /media/      → api      (permission-checked file streaming)
 ```
 
+The proxy forwards `/collab` without stripping the prefix, so the collab
+service answers its health probe at `/collab/healthz` externally and at
+`/healthz` for the container-internal Docker healthcheck.
+
 Self-hosters without a proxy can enable the optional `caddy` Compose profile
 (bundled Caddy with automatic TLS).
 
diff --git a/packages/shared/src/env.ts b/packages/shared/src/env.ts
index 1001d7e..b286ed8 100644
--- a/packages/shared/src/env.ts
+++ b/packages/shared/src/env.ts
@@ -6,19 +6,25 @@ import { z } from 'zod';
  * and crashes with a readable list of problems instead of failing later at
  * first use.
  */
+/** Fields every service configures the same way; keep these in sync. */
+const nodeEnv = z.enum(['development', 'test', 'production']).default('development');
+const logLevel = z.enum(['fatal', 'error', 'warn', 'info', 'debug', 'trace']).default('info');
+/** Version shown in health responses; injected at image build time. */
+const appVersion = z.string().default('0.0.0-dev');
+/** PostgreSQL connection string — required, there is no sensible default. */
+const databaseUrl = z
+  .string()
+  .min(1)
+  .refine((url) => url.startsWith('postgresql://') || url.startsWith('postgres://'), {
+    message: 'must be a postgresql:// connection string',
+  });
+
 export const apiEnvSchema = z.object({
-  NODE_ENV: z.enum(['development', 'test', 'production']).default('development'),
+  NODE_ENV: nodeEnv,
   PORT: z.coerce.number().int().min(1).max(65535).default(3000),
-  LOG_LEVEL: z.enum(['fatal', 'error', 'warn', 'info', 'debug', 'trace']).default('info'),
-  /** Version shown in health responses; injected at image build time. */
-  APP_VERSION: z.string().default('0.0.0-dev'),
-  /** PostgreSQL connection string — required, there is no sensible default. */
-  DATABASE_URL: z
-    .string()
-    .min(1)
-    .refine((url) => url.startsWith('postgresql://') || url.startsWith('postgres://'), {
-      message: 'must be a postgresql:// connection string',
-    }),
+  LOG_LEVEL: logLevel,
+  APP_VERSION: appVersion,
+  DATABASE_URL: databaseUrl,
   /** Set to "false" to skip `prisma migrate deploy` at startup (tests, tooling). */
   MIGRATE_ON_START: z
     .enum(['true', 'false'])
@@ -51,6 +57,21 @@ export const apiEnvSchema = z.object({
 
 export type ApiEnv = z.infer<typeof apiEnvSchema>;
 
+/**
+ * Configuration for the collaboration server (Hocuspocus, ADR 0003). It is a
+ * thin real-time front-end to the same PostgreSQL database as the api; it does
+ * not run migrations (the api owns the schema) and needs no SMTP or uploads.
+ */
+export const collabEnvSchema = z.object({
+  NODE_ENV: nodeEnv,
+  PORT: z.coerce.number().int().min(1).max(65535).default(3000),
+  LOG_LEVEL: logLevel,
+  APP_VERSION: appVersion,
+  DATABASE_URL: databaseUrl,
+});
+
+export type CollabEnv = z.infer<typeof collabEnvSchema>;
+
 export function parseEnv<Schema extends z.ZodTypeAny>(
   schema: Schema,
   env: Record<string, string | undefined>,
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index a6985ec..9e78196 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -143,7 +143,29 @@ importers:
         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':
+        specifier: workspace:*
+        version: link:../../packages/shared
+      '@hocuspocus/server':
+        specifier: ^4.3.0
+        version: 4.3.0(y-protocols@1.0.7(yjs@13.6.31))(yjs@13.6.31)
+      pg:
+        specifier: ^8.16.0
+        version: 8.22.0
+      pino:
+        specifier: ^9.6.0
+        version: 9.14.0
     devDependencies:
+      '@types/node':
+        specifier: ^26.1.0
+        version: 26.1.0
+      '@types/pg':
+        specifier: ^8.11.0
+        version: 8.20.0
+      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)
@@ -946,6 +968,16 @@ packages:
   '@floating-ui/utils@0.2.11':
     resolution: {integrity: sha512-RiB/yIh78pcIxl6lLMG0CgBXAZ2Y0eVHqMPYugu+9U0AeT6YBeiJpf7lbdJNIugFP5SIjwNRgo4DhR1Qxi26Gg==}
 
+  '@hocuspocus/common@4.3.0':
+    resolution: {integrity: sha512-8USsMvso01aMKOSSyvb8oTAlfES/nBM+Y74XjW5Fy5ovmOaVpoRRBrktIrEVwydrp8Cr6C66ivc0orcW/3P+Kg==}
+
+  '@hocuspocus/server@4.3.0':
+    resolution: {integrity: sha512-GX9ohUJAr6aIa2ewS0EPeyf3w12wLCBBSGfc76ZMOuZJZPUzDH6BJWFjkVlaVW3OmwZ5+jxdXXFUf6sAi4EIrg==}
+    engines: {node: '>=22'}
+    peerDependencies:
+      y-protocols: ^1.0.6
+      yjs: ^13.6.8
+
   '@hookform/resolvers@5.4.0':
     resolution: {integrity: sha512-EIsqr/t/qbinPIhGjMdtvutIN1Kk4uwbROE9/UQ93CAVGR7GkA7Y92+fX80OzXi/OB67jVFYwKGO1WzkxmkFZw==}
     peerDependencies:
@@ -1645,6 +1677,9 @@ packages:
   '@types/nodemailer@8.0.1':
     resolution: {integrity: sha512-PxpaInm8V1JQDd4j0ds5HfvWQk8JupS1C0Picb96QJsrrRDjBH+DlK7L4ZdNSqNULhiZRQHc40nLVShaGxXAMw==}
 
+  '@types/pg@8.20.0':
+    resolution: {integrity: sha512-bEPFOaMAHTEP1EzpvHTbmwR8UsFyHSKsRisLIHVMXnpNefSbGA1bD6CVy+qKjGSqmZqNqBDV2azOBo8TgkcVow==}
+
   '@types/qs@6.15.1':
     resolution: {integrity: sha512-GZHUBZR9hckSUhrxmp1nG6NwdpM9fCunJwyThLW1X3AyHgd9IlHb6VANpQQqDr2o/qQp6McZ3y/IA2rVzKzSbw==}
 
@@ -1917,6 +1952,9 @@ packages:
     resolution: {integrity: sha512-Izi8RQcffqCeNVgFigKli1ssklIbpHnCYc6AknXGYoB6grJqyeby7jv12JUQgmTAnIDnbck1uxksT4dzN3PWBA==}
     engines: {node: '>=12'}
 
+  async-mutex@0.5.0:
+    resolution: {integrity: sha512-1A94B18jkJ3DYq284ohPxoXbfTA5HsQ7/Mf4DEhcyLx3Bz27Rh59iScbB6EPiP+B+joue6YCxcMXSbFC1tZKwA==}
+
   asynckit@0.4.0:
     resolution: {integrity: sha512-Oei9OH4tRh0YqU3GxhX79dM/mwVgvbZJaSNaRk+bshkj0S5cfHcgYakreBjrHwatXKbz+IoIdYLxrKim2MjW0Q==}
 
@@ -2158,6 +2196,14 @@ packages:
     resolution: {integrity: sha512-uV2QOWP2nWzsy2aMp8aRibhi9dlzF5Hgh5SHaB9OiTGEyDTiJJyx0uy51QXdyWbtAHNua4XJzUKca3OzKUd3vA==}
     engines: {node: '>= 8'}
 
+  crossws@0.4.9:
+    resolution: {integrity: sha512-iWx+1OMSG2aOHpjyf9AESOzkwsVdS49cXM9dVrI2PDhxU5l2RIWE/KG56gk4BbAnsMoycvniJ9OnOxO9LRzHVA==}
+    peerDependencies:
+      srvx: '>=0.11.5'
+    peerDependenciesMeta:
+      srvx:
+        optional: true
+
   cssstyle@4.6.0:
     resolution: {integrity: sha512-2z+rWdzbbSZv6/rhtvzvqeZQHrBaqgogqt85sqFNbabZOuFbCVFb8kPeEtZjiKkbrm395irpNKiYeFeLiQnFPg==}
     engines: {node: '>=18'}
@@ -2740,6 +2786,10 @@ packages:
   keyv@4.5.4:
     resolution: {integrity: sha512-oxVHkHR/EJf2CNXnWxRLW6mg7JyCCUcG0DtEGmL2ctUo1PNTin1PUil+r/+4r5MpVgC/fn1kjsx7mjSujKqIpw==}
 
+  kleur@4.1.5:
+    resolution: {integrity: sha512-o+NO+8WrRiQEE4/7nwRJhN1HWpVmJm511pBHUxPLtp0BUISzlBplORYSmTclCnJvQq2tKu/sgl3xVpkc7ZWuQQ==}
+    engines: {node: '>=6'}
+
   levn@0.4.1:
     resolution: {integrity: sha512-+bT2uH4E5LGE7h/n3evcS/sQlJXCpIp6ym8OWJ5eV6+67Dsql/LaaT7qJBAt2rzfoa/5QBGBhxDix1dMt2kQKQ==}
     engines: {node: '>= 0.8.0'}
@@ -3040,6 +3090,40 @@ packages:
   perfect-debounce@1.0.0:
     resolution: {integrity: sha512-xCy9V055GLEqoFaHoC1SoLIaLmWctgCUaBaWxDZ7/Zx4CTyX7cJQLJOok/orfjZAh9kEYpjJa4d0KcJmCbctZA==}
 
+  pg-cloudflare@1.4.0:
+    resolution: {integrity: sha512-Vo7z/6rrQYxpNRylp4Tlob2elzbh+N/MOQbxFVWCxS7oEx6jF53GTJFxK2WWpKuBRkmiin4Mt+xofFDjx09R0A==}
+
+  pg-connection-string@2.14.0:
+    resolution: {integrity: sha512-XwWDGcLRGCXAR8F/AM5bG7Q+A3Wm2s6QeEjlOKZLlH3UYcguiqCWKyWXVag5TLTIjR7oOJUY8kcADaZgWPyLeg==}
+
+  pg-int8@1.0.1:
+    resolution: {integrity: sha512-WCtabS6t3c8SkpDBUlb1kjOs7l66xsGdKpIPZsg4wR+B3+u9UAum2odSsF9tnvxg80h4ZxLWMy4pRjOsFIqQpw==}
+    engines: {node: '>=4.0.0'}
+
+  pg-pool@3.14.0:
+    resolution: {integrity: sha512-gKtPkFdQPU3DksooVLi9LsjZxrsBUZIpa+7aVx+LV5pNh0KzP4Zleud2po+ConrxbuXGBJ6Hfer6hdgpIBpBaw==}
+    peerDependencies:
+      pg: '>=8.0'
+
+  pg-protocol@1.15.0:
+    resolution: {integrity: sha512-cq9sECI5s0+uPUXjbz8ioyPJni6RzsRib0US67i5IoTZKw8fNeYlVE7u8F4dG7vEJJtc5wdD1K189lCCUwqWTQ==}
+
+  pg-types@2.2.0:
+    resolution: {integrity: sha512-qTAAlrEsl8s4OiEQY69wDvcMIdQN6wdz5ojQiOy6YRMuynxenON0O5oCpJI6lshc6scgAY8qvJ2On/p+CXY0GA==}
+    engines: {node: '>=4'}
+
+  pg@8.22.0:
+    resolution: {integrity: sha512-8wih1vVIBMxoUM2oB4soJsD9tDnDpLv4OXBJ+EJzFsvycD+lfyIreC2gGHq78f8jbLLt+bvlPTFdFZfJkOuzAA==}
+    engines: {node: '>= 16.0.0'}
+    peerDependencies:
+      pg-native: '>=3.0.1'
+    peerDependenciesMeta:
+      pg-native:
+        optional: true
+
+  pgpass@1.0.5:
+    resolution: {integrity: sha512-FdW9r/jQZhSeohs1Z3sI1yxFQNFvMcnmfuj4WBMUTxOrAyLMaTcE1aAMBiTlbMNaXvBCQuVi0R7hd8udDSP7ug==}
+
   picocolors@1.1.1:
     resolution: {integrity: sha512-xceH2snhtb5M9liqDsmEw56le376mTZkEX/jEb/RxNFyegNul7eNslCXP9FDj/Lcu0X8KEyMceP2ntpaHrDEVA==}
 
@@ -3117,6 +3201,22 @@ packages:
     resolution: {integrity: sha512-vuwillviilfKZsg0VGj5R/YwwcHx4SLsIOI/7K6mQkWx+l5cUHTjj5g0AasTBcyXsbfTgrwsUNmVUb5xVwyPwg==}
     engines: {node: ^10 || ^12 || >=14}
 
+  postgres-array@2.0.0:
+    resolution: {integrity: sha512-VpZrUqU5A69eQyW2c5CA1jtLecCsN2U/bD6VilrFDWq5+5UIEVO7nazS3TEcHf1zuPYO/sqGvUvW62g86RXZuA==}
+    engines: {node: '>=4'}
+
+  postgres-bytea@1.0.1:
+    resolution: {integrity: sha512-5+5HqXnsZPE65IJZSMkZtURARZelel2oXUEO8rH83VS/hxH5vv1uHquPg5wZs8yMAfdv971IU+kcPUczi7NVBQ==}
+    engines: {node: '>=0.10.0'}
+
+  postgres-date@1.0.7:
+    resolution: {integrity: sha512-suDmjLVQg78nMK2UZ454hAG+OAW+HQPZ6n++TNDUX+L0+uUlLywnoxJKDou51Zm+zTCjrCl0Nq6J9C5hP9vK/Q==}
+    engines: {node: '>=0.10.0'}
+
+  postgres-interval@1.2.0:
+    resolution: {integrity: sha512-9ZhXKM/rw350N1ovuWHbGxnGh/SNJ4cnxHiM0rxE4VN41wsg8P8zWn9hv/buK00RP4WvlOyr/RBDiptyxVbkZQ==}
+    engines: {node: '>=0.10.0'}
+
   prelude-ls@1.2.1:
     resolution: {integrity: sha512-vkcDPrRZo1QZLbn5RLGPpg/WmIQ65qoWWhcGKf/b5eplkkarX0m9z8ppCat4mlOqUsWpyNuYgO3VRyrYHSzX5g==}
     engines: {node: '>= 0.8.0'}
@@ -3954,6 +4054,10 @@ packages:
   xmlchars@2.2.0:
     resolution: {integrity: sha512-JZnDKK8B0RCDw84FNdDAIpZK+JuJw+s7Lz8nksI7SIuU3UXJJslUthsi+uWBUYOwPFwW7W7PRLRfUKpxjtjFCw==}
 
+  xtend@4.0.2:
+    resolution: {integrity: sha512-LKYU1iAXJXUgAXn9URjiu+MWhyUXHsvfp7mcuYm9dSUKK0/CjtrUwFAxD82/mCWbtLsGjFIad0wIsod4zrTAEQ==}
+    engines: {node: '>=0.4'}
+
   y-prosemirror@1.3.7:
     resolution: {integrity: sha512-NpM99WSdD4Fx4if5xOMDpPtU3oAmTSjlzh5U4353ABbRHl1HtAFUx6HlebLZfyFxXN9jzKMDkVbcRjqOZVkYQg==}
     engines: {node: '>=16.0.0', npm: '>=8.0.0'}
@@ -4491,6 +4595,22 @@ snapshots:
 
   '@floating-ui/utils@0.2.11': {}
 
+  '@hocuspocus/common@4.3.0':
+    dependencies:
+      lib0: 0.2.117
+
+  '@hocuspocus/server@4.3.0(y-protocols@1.0.7(yjs@13.6.31))(yjs@13.6.31)':
+    dependencies:
+      '@hocuspocus/common': 4.3.0
+      async-mutex: 0.5.0
+      crossws: 0.4.9
+      kleur: 4.1.5
+      lib0: 0.2.117
+      y-protocols: 1.0.7(yjs@13.6.31)
+      yjs: 13.6.31
+    transitivePeerDependencies:
+      - srvx
+
   '@hookform/resolvers@5.4.0(react-hook-form@7.80.0(react@19.2.7))':
     dependencies:
       '@standard-schema/utils': 0.3.0
@@ -5149,6 +5269,12 @@ snapshots:
     dependencies:
       '@types/node': 26.1.0
 
+  '@types/pg@8.20.0':
+    dependencies:
+      '@types/node': 26.1.0
+      pg-protocol: 1.15.0
+      pg-types: 2.2.0
+
   '@types/qs@6.15.1': {}
 
   '@types/range-parser@1.2.7': {}
@@ -5493,6 +5619,10 @@ snapshots:
 
   assertion-error@2.0.1: {}
 
+  async-mutex@0.5.0:
+    dependencies:
+      tslib: 2.8.1
+
   asynckit@0.4.0: {}
 
   atomic-sleep@1.0.0: {}
@@ -5722,6 +5852,8 @@ snapshots:
       shebang-command: 2.0.0
       which: 2.0.2
 
+  crossws@0.4.9: {}
+
   cssstyle@4.6.0:
     dependencies:
       '@asamuzakjp/css-color': 3.2.0
@@ -6387,6 +6519,8 @@ snapshots:
     dependencies:
       json-buffer: 3.0.1
 
+  kleur@4.1.5: {}
+
   levn@0.4.1:
     dependencies:
       prelude-ls: 1.2.1
@@ -6646,6 +6780,41 @@ snapshots:
 
   perfect-debounce@1.0.0: {}
 
+  pg-cloudflare@1.4.0:
+    optional: true
+
+  pg-connection-string@2.14.0: {}
+
+  pg-int8@1.0.1: {}
+
+  pg-pool@3.14.0(pg@8.22.0):
+    dependencies:
+      pg: 8.22.0
+
+  pg-protocol@1.15.0: {}
+
+  pg-types@2.2.0:
+    dependencies:
+      pg-int8: 1.0.1
+      postgres-array: 2.0.0
+      postgres-bytea: 1.0.1
+      postgres-date: 1.0.7
+      postgres-interval: 1.2.0
+
+  pg@8.22.0:
+    dependencies:
+      pg-connection-string: 2.14.0
+      pg-pool: 3.14.0(pg@8.22.0)
+      pg-protocol: 1.15.0
+      pg-types: 2.2.0
+      pgpass: 1.0.5
+    optionalDependencies:
+      pg-cloudflare: 1.4.0
+
+  pgpass@1.0.5:
+    dependencies:
+      split2: 4.2.0
+
   picocolors@1.1.1: {}
 
   picomatch@4.0.4: {}
@@ -6737,6 +6906,16 @@ snapshots:
       picocolors: 1.1.1
       source-map-js: 1.2.1
 
+  postgres-array@2.0.0: {}
+
+  postgres-bytea@1.0.1: {}
+
+  postgres-date@1.0.7: {}
+
+  postgres-interval@1.2.0:
+    dependencies:
+      xtend: 4.0.2
+
   prelude-ls@1.2.1: {}
 
   prettier@3.9.4: {}
@@ -7583,6 +7762,8 @@ snapshots:
 
   xmlchars@2.2.0: {}
 
+  xtend@4.0.2: {}
+
   y-prosemirror@1.3.7(prosemirror-model@1.25.9)(prosemirror-state@1.4.4)(prosemirror-view@1.42.0)(y-protocols@1.0.7(yjs@13.6.31))(yjs@13.6.31):
     dependencies:
       lib0: 0.2.117