feat: add department_id to all business entities

apps/server/src/entities/classroom-rental.entity.ts

@@ -10,6 +10,7 @@ import {
 } from 'typeorm';
 import { Classroom } from './classroom.entity';
 import { Tenant } from './tenant.entity';
+import { Department } from './department.entity';
 
 @Entity('classroom_rentals')
 @Index(['classroomId', 'startDate', 'endDate'])
@@ -64,4 +65,11 @@ export class ClassroomRental {
 
   @UpdateDateColumn({ name: 'updated_at' })
   updatedAt: Date;
+
+  @Column({ name: 'department_id', type: 'integer', nullable: true })
+  departmentId: number;
+
+  @ManyToOne(() => Department, { nullable: true })
+  @JoinColumn({ name: 'department_id' })
+  department: Department;
 }