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/index.ts

@@ -8,6 +8,7 @@ export { BillItem } from './bill-item.entity';
 export { User } from './user.entity';
 export { OperationLog } from './operation-log.entity';
 export { Deposit } from './deposit.entity';
+export { DepositInstallment } from './deposit-installment.entity';
 export { Classroom } from './classroom.entity';
 export { Tenant } from './tenant.entity';
 export { ClassroomRental } from './classroom-rental.entity';