Root cause of flag not working in staging:
- WorkerSettings had no 'redis_url' field, so REDIS_URL env var was ignored
- RenderEngineResolver fallback to broker_url (default redis://redis:6379/0)
- Staging redis hostname is xiaoxia-redis-staging, so connection failed
- Resolver degraded to in-memory store, flag always defaulted to disabled (legacy)
Fix: add redis_url field so REDIS_URL env var is properly mapped