refactor: remove obsolete scaffolding and dead modules

Drop the unused root hello-world controller, empty CommonModule imports, development seeding code, legacy student report entity, stale DTOs, notification hook, and their placeholder tests.

apps/server/src/expenses/expenses.module.ts

@@ -1,6 +1,5 @@
 import { Module } from '@nestjs/common';
 import { TypeOrmModule } from '@nestjs/typeorm';
-import { CommonModule } from '../common/common.module';
 import { RoomExpense } from '../entities/room-expense.entity';
 import { PersonalExpense } from '../entities/personal-expense.entity';
 import { Room } from '../entities/room.entity';
@@ -13,7 +12,6 @@ import { OperationLogsModule } from '../operation-logs/operation-logs.module';
   imports: [
     TypeOrmModule.forFeature([RoomExpense, PersonalExpense, Room, Student]),
     OperationLogsModule,
-    CommonModule,
   ],
   controllers: [ExpensesController],
   providers: [ExpensesService],