feat: integrate notification creation into business modules (classes, schedules done; bills/occupancies/deposits need student→userId mapping)

apps/server/src/bills/bills.module.ts

@@ -1,4 +1,5 @@
 import { Module } from '@nestjs/common';
+import { NotificationsModule } from '../notifications/notifications.module';
 import { TypeOrmModule } from '@nestjs/typeorm';
 import { Bill } from '../entities/bill.entity';
 import { BillItem } from '../entities/bill-item.entity';
@@ -22,6 +23,7 @@ import { BillsController } from './bills.controller';
       Room,
       Deposit,
     ]),
+    NotificationsModule,
   ],
   controllers: [BillsController],
   providers: [BillsService, BillsExportService],