chore(openspec): archive add-shop-coupons

Sync the shop-coupons capability plus the order, frontend-mall, and
frontend-shop-admin requirements into the main specs, and write the capability
Purpose the archive leaves as a placeholder.

Renaming the coupon scenario in the flash-sales and group-buying order deltas
to the exact name now in the main order spec keeps those changes archivable,
since a MODIFIED requirement replaces the whole block.

openspec/changes/archive/2026-09-18-add-shop-coupons/design.md

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+## Context
+
+Checkout already splits a Redis cart into one database order per shop, converts SKU prices to the buyer-selected currency, and locks SKU rows. Mall coupon pages still read fixtures. The new capability crosses merchant management, customer claim/listing, and order creation.
+
+## Goals / Non-Goals
+
+**Goals:**
+- Make shop coupons live, auditable, scoped, and safe under concurrent claims and checkout.
+- Let a customer select no more than one coupon per generated shop order.
+- Persist the realized minor-unit discount on the order.
+
+**Non-Goals:**
+- Platform coupons, threshold reductions, customer targeting, or composing coupons with activity prices in this change. Later flash-sale and group-buying changes reject coupons on activity-priced shop orders.
+
+## Decisions
+
+### Coupon data is template plus owned snapshot
+`coupon_templates` owns `shop_id`, localized title, amount/threshold minor, currency, claim stock, active window, and enabled state. `coupons` owns `user_id`, `template_id`, `shop_id`, copied terms/window, `status`, and nullable `order_id`. A unique `(user_id, template_id)` prevents duplicate claims. The snapshot preserves a claim if a template is edited or disabled later.
+
+### Checkout receives choices, not money
+The shared checkout body adds `coupon_by_shop: Record<shop UUID, coupon UUID>`. The server rejects an unknown shop, a coupon not owned by the customer or shop, an unavailable or expired coupon, or an unmet threshold. It converts coupon amount and threshold to the buyer-selected order currency with the same enabled-currency conversion used for SKU prices. The server stores `coupon_id` and `discount_minor`; clients never submit a discount.
+
+### One transaction protects stock and redemption
+Checkout locks SKU rows in UUID order, then selected coupon rows in UUID order. It conditionally decrements template claim stock on claim and moves a claimed coupon to redeemed only while all eligibility predicates hold. Pending-payment cancellation restores the coupon in the same transaction as SKU stock restoration. Expiry is evaluated at claim/list/checkout and recorded lazily; no scheduler is required.
+
+### Ownership and surfaces follow existing boundaries
+Customer routes use `AuthUser`; shop routes require shop roles and `own_shop`. The contract remains exclusively in `@vmall/shared`; Mall replaces direct fixture imports and shop-admin gains template CRUD plus a nav entry. Checkout, payment, and order-detail surfaces show the server `discount_minor`; clients never invent it.
+
+### Later activities do not stack with coupons
+This change is coupon-only: threshold and discount use the shop merchandise subtotal after catalog (or converted) line prices. When a later change introduces flash-sale or group-buy line prices, checkout MUST reject `coupon_by_shop` for any generated shop order that applied an activity price. Subsequent `order` spec deltas MUST copy this change's coupon validation, persistence, and pending-payment restore clauses instead of replacing the whole checkout requirement with activity-only text.
+
+## Risks / Trade-offs
+
+- Exchange-rate changes can change an unredeemed coupon's converted value; the realized `discount_minor` is immutable once ordered. Fixing value across currencies would need a new rate-snapshot policy.
+- Activity-priced shop orders reject coupons rather than stack. Threshold reductions remain unspecified until a later change.
+- Checkout contract expansion requires the fixed-data adapter to implement the same surface.