feat(config): add MosaicConfig schema + loader with tier auto-detection

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/gateway/src/app.module.ts

@@ -1,6 +1,7 @@
 import { Module } from '@nestjs/common';
 import { APP_GUARD } from '@nestjs/core';
 import { HealthController } from './health/health.controller.js';
+import { ConfigModule } from './config/config.module.js';
 import { DatabaseModule } from './database/database.module.js';
 import { AuthModule } from './auth/auth.module.js';
 import { BrainModule } from './brain/brain.module.js';
@@ -28,6 +29,7 @@ import { ThrottlerGuard, ThrottlerModule } from '@nestjs/throttler';
 @Module({
   imports: [
     ThrottlerModule.forRoot([{ name: 'default', ttl: 60_000, limit: 60 }]),
+    ConfigModule,
     DatabaseModule,
     AuthModule,
     BrainModule,