feat: 优化项目目录结构

apps/backend/src/notification/notification.module.ts

@@ -0,0 +1,8 @@
+import { Module } from '@nestjs/common';
+import { NotificationService } from './notification.service';
+
+@Module({
+  providers: [NotificationService],
+  exports: [NotificationService],
+})
+export class NotificationModule {}