feat: add NotificationsService with SSE subject pool

apps/server/src/notifications/notifications.module.ts

@@ -0,0 +1,11 @@
+import { Module } from '@nestjs/common';
+import { TypeOrmModule } from '@nestjs/typeorm';
+import { Notification } from '../entities/notification.entity';
+import { NotificationsService } from './notifications.service';
+
+@Module({
+  imports: [TypeOrmModule.forFeature([Notification])],
+  providers: [NotificationsService],
+  exports: [NotificationsService],
+})
+export class NotificationsModule {}