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>

frontend/Dockerfile

@@ -13,6 +13,10 @@ RUN npm ci
 COPY . .
 
 # Build arguments (set at build time)
+# NOTE: VITE_SELF_SERVE_ENABLED is only a network-error fallback — the live
+# toggle is the backend /config/public flag; rebuilding with a new value here
+# does NOT flip self-serve. VITE_STRIPE_PUBLISHABLE_KEY is currently unused
+# (checkout is fully backend-driven); kept for a future client-side Stripe.js.
 ARG VITE_API_URL
 ARG VITE_SENTRY_DSN
 ARG VITE_PUBLIC_POSTHOG_KEY