✨ feat: add mana-api-gateway for monetizing core services

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.

services/mana-api-gateway/.env.example

@@ -0,0 +1,34 @@
+# Server
+PORT=3030
+NODE_ENV=development
+
+# Database (same as mana-core-auth)
+DATABASE_URL=postgresql://manacore:devpassword@localhost:5432/manacore
+
+# Redis
+REDIS_HOST=localhost
+REDIS_PORT=6379
+REDIS_PASSWORD=
+
+# Backend Services
+SEARCH_SERVICE_URL=http://localhost:3021
+STT_SERVICE_URL=http://localhost:3020
+TTS_SERVICE_URL=http://localhost:3022
+
+# Auth Service (for JWT validation & credits)
+MANA_CORE_AUTH_URL=http://localhost:3001
+
+# API Key Generation
+API_KEY_PREFIX_LIVE=sk_live_
+API_KEY_PREFIX_TEST=sk_test_
+
+# Rate Limiting Defaults
+DEFAULT_RATE_LIMIT=10
+DEFAULT_MONTHLY_CREDITS=100
+
+# CORS
+CORS_ORIGINS=http://localhost:3000,http://localhost:5173
+
+# Development Auth Bypass
+DEV_BYPASS_AUTH=true
+DEV_USER_ID=00000000-0000-0000-0000-000000000000