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/app.module.ts

@@ -61,7 +61,6 @@ import { SchedulesModule } from './schedules/schedules.module';
 import { ClassroomRentalsModule } from './classroom-rentals/classroom-rentals.module';
 import { SyncModule } from './sync/sync.module';
 import { NotificationsModule } from './notifications/notifications.module';
-import { CommonModule } from './common/common.module';
 import { ArchiveModule } from './archive/archive.module';
 import { ExpenseTypesModule } from './expense-types/expense-types.module';
 
@@ -161,7 +160,6 @@ import { IntegrationConfigModule } from './integration/config/config.module';
     ClassroomRentalsModule,
     SyncModule,
     NotificationsModule,
-    CommonModule,
     ArchiveModule,
     IntegrationConfigModule,
     ExpenseTypesModule,