feat: add department_id to all business entities

apps/server/src/entities/bill.entity.ts

@@ -9,6 +9,7 @@ import {
 } from 'typeorm';
 import { Student } from './student.entity';
 import { BillItem } from './bill-item.entity';
+import { Department } from './department.entity';
 
 @Entity('bills')
 export class Bill {
@@ -45,4 +46,11 @@ export class Bill {
 
   @OneToMany(() => BillItem, (bi) => bi.bill)
   items: BillItem[];
+
+  @Column({ name: 'department_id', type: 'integer', nullable: true })
+  departmentId: number;
+
+  @ManyToOne(() => Department, { nullable: true })
+  @JoinColumn({ name: 'department_id' })
+  department: Department;
 }