fix(prod): harden production configuration for launch

- Gate Swagger/ReDoc/OpenAPI behind DEBUG (no public API schema in prod)
- Sentry send_default_pii only in dev (no auth headers/bodies in events)
- Remove alembic from Dockerfile CMD (releaseCommand owns migrations; CMD copy raced across replicas/restarts)
- Decouple rate limiting from DEBUG via RATE_LIMIT_ENABLED (PR envs with DEBUG=true were unlimited); tests disable the live limiter in conftest
- max_instances=1 on the 4 scheduler jobs missing it
- Boot-time failure when SELF_SERVE_ENABLED without RESEND_API_KEY/ANTHROPIC_API_KEY/FRONTEND_URL
- Reject localhost OAUTH_REDIRECT_BASE outside DEBUG
- pool_pre_ping + pool_recycle on the app engine
- Frontend: DEV-gate stale-async console.warn; document VITE_SELF_SERVE_ENABLED fallback semantics in Dockerfile

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/core/database.py

@@ -7,7 +7,10 @@ from app.core.tenant_context import register_tenant_listener
 engine = create_async_engine(
     settings.DATABASE_URL,
     echo=settings.DEBUG,
-    future=True
+    future=True,
+    # Detect connections dropped by DB restarts/maintenance instead of failing requests
+    pool_pre_ping=True,
+    pool_recycle=1800,
 )
 
 # Create async session factory