#197: security response headers and an explicitly restrictive CORS policy

Hand-rolled middleware instead of helmet: the header set is small enough
to own, every value is a deliberate decision, and the api gains no
transitive dependency. HSTS (no includeSubDomains — the api cannot speak
for sibling subdomains), nosniff, Referrer-Policy no-referrer,
X-Frame-Options SAMEORIGIN (not DENY: the plugin sandbox frame embeds
same-origin and its CSP has no frame-ancestors, so this header governs),
and a minimal deny-all Permissions-Policy.

CORS grants no foreign origin anything; only the APP_BASE_URL origin is
ever echoed (where browsers do not consult CORS anyway), with
Vary: Origin on every response. No preflight handling — same-origin
requests never preflight, and cross-origin API access is cookie-less by
design (PAT/Bearer).

Wired via the AppModule MiddlewareConsumer so createTestApp boots the
identical middleware. Fences: security-headers.e2e.test.ts (header set,
foreign origin gets no ACAO) and a frame assertion in
plugins.e2e.db.test.ts (framing stays possible). Rationale table in
security.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0168Ph5uBmHm8X28CSVpbpnJ

docs/vs-nfd/20-massnahmenplan.md

@@ -104,7 +104,7 @@ chain`_
       oder entfernen · 2 AT · #194
 - [x] **Papierkorb aus dem Suchindex** entfernen statt query-seitig filtern · 2 AT · #195
 - [x] **Retention-Job für `audit_log`** · 1 AT · #196
-- [ ] **Security-Header** (helmet), CORS explizit restriktiv · 1 AT · #197
+- [x] **Security-Header** (helmet), CORS explizit restriktiv · 1 AT · #197
 - [ ] **SBOM in CI** (CycloneDX/syft) + Lizenzreport als Artefakt · 1–2 AT · #202
 - [ ] `deploy/compose/.env` prüfen, Beispieldatei statt Realdatei · 0,5 AT · #198
 - [ ] **Attachment-Integritätshashes** · +2–3 AT · #199 ⟵ neu aus Roadmap