Implement custom NestJS API Gateway for mana-search, mana-stt, and mana-tts:
- API Key management with CRUD operations and key regeneration
- Redis-based sliding window rate limiting
- Credit-based billing with tier support (free, pro, enterprise)
- Usage tracking with daily aggregates
- Proxy services to backend microservices
- Prometheus metrics endpoint
- JWT auth for management API, API key auth for public API
Database schema uses separate `api_gateway` schema in shared manacore DB.
