vmall/openspec/changes/add-shop-coupons/tasks.md

1. Coupon domain and contract

• 1.1 Add additive Postgres migrations for coupon templates, owned coupon snapshots, order coupon binding, and realized discount minor values.
• 1.2 Implement the Rust coupon module (DTOs, repository, service, handlers, router registration) with customer and shop RBAC/ownership boundaries.
• 1.3 Add @vmall/shared coupon types, API-client methods, localized UI strings, and matching fixed-data adapter methods.

2. Atomic claims and checkout redemption

• 2.1 Implement conditional template stock claim and per-customer duplicate protection with valid-window checks.
• 2.2 Extend checkout request handling with per-shop coupon choices, server-side currency conversion and threshold validation, and persisted discounts.
• 2.3 Restore redeemed coupons atomically with stock when a pending-payment order is cancelled.
• 2.4 Add API integration coverage for claim contention, cross-shop/expired/ineligible rejection, multi-shop redemption, and cancellation restoration.

3. Merchant and customer surfaces

• 3.1 Add shop-admin coupon-template management and a coupons nav entry scoped to the authenticated shop.
• 3.2 Replace mall product-detail coupon strip, buyer coupon list, and checkout coupon selection with shared live-client calls; show persisted discount on payment and order-detail views.
• 3.3 Remove the affected direct coupon fixture imports while retaining the complete fixed-data adapter rollback surface.

4. Verification and specification

• 4.1 Seed deterministic coupon templates and verify the live claim-to-checkout-to-cancel flow against the API.
• 4.2 Run cargo test for vmall-api, builds for mall and shop-admin, and strict validation for this OpenSpec change.