feat: Implement Phase 6 Shopping and Checkout

- Introduced authenticated shopping state and atomic checkout process without creating orders or payments.
- Reused existing components from previous phases including identities, addresses, catalogues, pricing, and inventory.
- Developed cart architecture to support one active cart per user with unique items per variant.
- Implemented dynamic cart pricing with various precedence rules for promotions and coupons.
- Created a wishlist feature that exposes only visible active products without reserving inventory.
- Integrated inventory validation to ensure availability of items before adding to cart.
- Developed a checkout architecture that locks the cart, revalidates pricing and shipping, and reserves inventory.
- Added shipping zones, methods, and rates with configurable options for international shipping and duty/tax boundaries.
- Implemented checkout snapshots to capture immutable checkout details.
- Introduced idempotency for checkout requests to prevent duplicate processing.
- Added functionality for coupon integration and business checkout validation.
- Established permissions for managing shipping zones, methods, and rates.
- Created comprehensive unit tests covering various aspects of the shopping and checkout processes.
- Added cron job for handling checkout expiry reconciliation.
- Created migration script to set up new database tables and constraints for carts, checkout sessions, and shipping.

tests/unit/shopping.permissions.test.js

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+const{checkPermission}=require("../../app/middleware/permission.middleware");describe("shipping administration security",()=>{test("customer cannot manage shipping",()=>{const req={user:{accountType:"CUSTOMER",permissions:[]},method:"POST"},res={status:jest.fn().mockReturnThis(),json:jest.fn()},next=jest.fn();checkPermission("shipping.rates.manage",{custom:true})(req,res,next);expect(res.status).toHaveBeenCalledWith(403);expect(next).not.toHaveBeenCalled();});});