✨ 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/src/app.module.ts

@@ -0,0 +1,27 @@
+import { Module } from '@nestjs/common';
+import { ConfigModule } from '@nestjs/config';
+import configuration from './config/configuration';
+import { DatabaseModule } from './db/database.module';
+import { HealthModule } from './health/health.module';
+import { ApiKeysModule } from './api-keys/api-keys.module';
+import { UsageModule } from './usage/usage.module';
+import { ProxyModule } from './proxy/proxy.module';
+import { CreditsModule } from './credits/credits.module';
+import { MetricsModule } from './metrics/metrics.module';
+
+@Module({
+	imports: [
+		ConfigModule.forRoot({
+			isGlobal: true,
+			load: [configuration],
+		}),
+		DatabaseModule,
+		HealthModule,
+		ApiKeysModule,
+		UsageModule,
+		ProxyModule,
+		CreditsModule,
+		MetricsModule,
+	],
+})
+export class AppModule {}