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/specs/shop-coupons/spec.md

@@ -0,0 +1,27 @@
+# shop-coupons Specification
+
+## Purpose
+Let a shop issue its own coupons and a customer claim and redeem them. A template owns the offer, window, and claim stock; claiming copies the terms into a customer-owned snapshot so later edits cannot rewrite a held coupon. Checkout redeems at most one owned coupon per generated shop order, applies a server-calculated discount, and restores the coupon when a pending-payment order is cancelled. Activity-priced shop orders reject coupons rather than stack.
+
+## Requirements
+
+### Requirement: Shop coupon template management
+A shop_owner or shop_staff SHALL create, list, update, disable, and delete only that shop's coupon templates. A template SHALL contain localized title, fixed `amount_minor`, `threshold_minor`, enabled ISO currency, non-negative claim stock, and an inclusive active window; amount MUST be positive and threshold MUST NOT be negative.
+
+#### Scenario: cross-shop template is hidden
+- **WHEN** a shop user requests or mutates another shop's coupon template
+- **THEN** the API returns 404
+
+### Requirement: Customer coupon claim and listing
+An authenticated customer SHALL list owned coupons and claim an active template at most once. Claiming SHALL atomically create a user-held snapshot and decrement claim stock only when stock remains; listing SHALL distinguish claimed, redeemed, and expired coupons.
+
+#### Scenario: concurrent final claim
+- **WHEN** two customers claim the final remaining template instance concurrently
+- **THEN** exactly one claim succeeds and the other receives 409
+
+### Requirement: Coupon redemption lifecycle
+A claimed coupon SHALL be redeemable only by its owner, for its issuing shop, within its snapshot window, and when the shop-order subtotal reaches its snapshot threshold. Redemption SHALL bind the coupon to one order; cancellation of that pending-payment order SHALL restore it to claimed. Later activity-priced shop orders (flash sale or group buy) SHALL NOT redeem a coupon; those changes restated this exclusion on checkout.
+
+#### Scenario: cancelled order restores coupon
+- **WHEN** a customer cancels a pending-payment order redeemed with a coupon
+- **THEN** the coupon again has claimed status and no order binding