feat(api): optional API-key auth middleware

Adds defence-in-depth shared-secret auth that activates when API_KEY
is set. Behaviour:

- empty API_KEY (dev default): every request allowed, middleware is
  not even installed;
- non-empty API_KEY: every request under APP_API_PREFIX except
  /health must carry X-API-Key: <value> or
  Authorization: Bearer <value>. /, /docs, /redoc, /openapi.json and
  CORS preflight stay open. hmac.compare_digest is used for the
  constant-time comparison.

The middleware resolves settings lazily so test fixtures can reload
app.config and have the new API_KEY take effect on the next install.

Tests (tests/test_api_security.py, 5 cases):
- /health remains open;
- protected route rejects missing key (401);
- protected route accepts X-API-Key header;
- protected route accepts Authorization: Bearer header;
- protected route rejects a wrong key.

Frontend:
- VITE_API_KEY env reads the key and Axios injects it on every
  request, falling back to no header when empty so SSO/reverse-proxy
  deployments stay unchanged.
- vite-env.d.ts adds the new env entry.

Docs/ops:
- .env.example documents the dev-default empty key;
- .env.prod.example marks API_KEY as a required rotation point;
- docker-compose.yml forwards API_KEY (defaults to empty);
- docker-compose.prod.yml fails the stack with ?:required when API_KEY
  is missing;
- RUNBOOK gains an API authentication section with header examples
  and the reverse-proxy + key layering recommendation.

pytest -q: 33 passed (5 new security + 28 prior).
npx tsc --noEmit: clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/main.py

@@ -10,6 +10,7 @@ from fastapi.middleware.cors import CORSMiddleware
 
 from app import __version__
 from app.api import routes_health, routes_ingestion, routes_search
+from app.api.security import install_api_key_auth
 from app.config import settings
 from app.logging_config import configure_logging, get_logger
 
@@ -43,9 +44,10 @@ app.add_middleware(
     allow_origins=settings.cors_origins,
     allow_credentials=True,
     allow_methods=["GET", "POST", "PUT", "PATCH", "DELETE", "OPTIONS"],
-    allow_headers=["*"],
+    allow_headers=["*", "X-API-Key", "Authorization"],
     max_age=3600,
 )
+install_api_key_auth(app)
 
 app.include_router(routes_health.router, prefix=settings.app_api_prefix)
 app.include_router(routes_ingestion.router, prefix=settings.app_api_prefix)