fix(infra): PostgreSQL → MySQL + tsconfig.build exclude frontend/

src/app.module.ts

@@ -22,7 +22,7 @@ import { HealthController } from './common/health.controller';
       imports: [ConfigModule],
       inject: [ConfigService],
       useFactory: (config: ConfigService) => ({
-        type: 'postgres',
+        type: 'mysql',
         url: config.get<string>('DATABASE_URL'),
         autoLoadEntities: true,
         synchronize: config.get('NODE_ENV') !== 'production',