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

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

apps/gateway/src/config/config.module.ts

@@ -0,0 +1,16 @@
+import { Global, Module } from '@nestjs/common';
+import { loadConfig, type MosaicConfig } from '@mosaic/config';
+
+export const MOSAIC_CONFIG = 'MOSAIC_CONFIG';
+
+@Global()
+@Module({
+  providers: [
+    {
+      provide: MOSAIC_CONFIG,
+      useFactory: (): MosaicConfig => loadConfig(),
+    },
+  ],
+  exports: [MOSAIC_CONFIG],
+})
+export class ConfigModule {}