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/.env.example

@@ -39,4 +39,6 @@ STRIPE_WEBHOOK_SECRET=whsec_
 # global flag for specific users — used for prod test-mode validation
 # before the public flip. Empty by default.
 SELF_SERVE_ENABLED=false
-INTERNAL_TESTER_EMAILS=
+INTERNAL_TESTER_EMAILS=
+# Rate limiting (decoupled from DEBUG; keep true in PR/staging/prod)
+RATE_LIMIT_ENABLED=false