feat: Implement return and refund functionality in commerce module

- Added return request handling in return.controller.js
- Implemented webhook handling for PayHere payments in webhook.controller.js
- Created models for coupon redemptions, invoices, orders, order items, payments, payment attempts, payment webhook events, refunds, return items, and return requests.
- Developed services for order management, payment processing, refunds, and returns.
- Introduced validation schemas for payment and return requests.
- Created migration scripts for new database tables related to orders, payments, refunds, and returns.
- Added unit tests for order state transitions and PayHere provider functionality.

tests/unit/commerce.validation.test.js

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+const s=require("../../app/validation/commerce.schemas"),returns=require("../../app/services/commerce/return.service");const input=body=>({body,params:{},query:{}});describe("Phase 7 input boundaries",()=>{test("customer return cannot submit refund amount",()=>expect(()=>s.returnRequest.parse(input({reasonCode:"DAMAGED",items:[{orderItemId:"i",quantity:1,resolution:"REFUND",amount:"999"}]}))).toThrow());test("refund quantities are positive integers",()=>expect(()=>s.refund.parse(input({paymentId:"p",reasonCode:"ADMIN",items:[{orderItemId:"i",quantity:0}]}))).toThrow());test("return window is centralized",()=>expect(returns.windowDays()).toBeGreaterThan(0));});