P2-14: Deposit installment tracking + refund approval flow

- Add DepositInstallment entity (id, depositId, amount, dueDate, paidDate, status, createdAt)
- Add installments OneToMany relation to Deposit entity with cascade+eager
- Add refund approval fields: refundStatus, refundRequestedAt, refundApprovedBy, refundApprovedAt
- Add installment DTOs (CreateInstallmentDto, UpdateInstallmentDto)
- Add refund approval DTOs (ApproveRefundDto, CreateDepositWithInstallmentsDto)
- Service: add/create/update/delete installments, requestRefund, approveRefund, findPendingRefunds
- Controller: GET deposits/:id, GET pending-refunds, POST :id/installments, PUT installments/:id, DELETE installments/:id, POST :id/request-refund, PUT :id/approve-refund
- Frontend: detail modal with installment list, refund request button, pending refunds tab with approve actions

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

@@ -4,9 +4,11 @@ import {
   Column,
   CreateDateColumn,
   ManyToOne,
+  OneToMany,
   JoinColumn,
 } from 'typeorm';
 import { Student } from './student.entity';
+import { DepositInstallment } from './deposit-installment.entity';
 
 @Entity('deposits')
 export class Deposit {
@@ -44,10 +46,26 @@ export class Deposit {
   @Column({ name: 'recorded_by', nullable: true })
   recordedBy: number;
 
+  @Column({ name: 'refund_status', length: 30, nullable: true })
+  refundStatus: string | null; // pending | head_teacher_approved | finance_approved | refunded
+
+  @Column({ name: 'refund_requested_at', nullable: true })
+  refundRequestedAt: Date | null;
+
+  @Column({ name: 'refund_approved_by', type: 'integer', nullable: true })
+  refundApprovedBy: number | null;
+
+  @Column({ name: 'refund_approved_at', nullable: true })
+  refundApprovedAt: Date | null;
+
+  @OneToMany(() => DepositInstallment, (i) => i.deposit, { cascade: true, eager: true })
+  installments: DepositInstallment[];
+
   @CreateDateColumn({ name: 'created_at' })
   createdAt: Date;
 
   @ManyToOne(() => Student, { eager: true })
   @JoinColumn({ name: 'student_id' })
   student: Student;
+
 }