docs(openspec): plan shop coupons, accounts, points, flash sales, and group buying

Capture exclusive promotion composition, empty-group cancel, and mall surface scope so the five marketing changes can be implemented without stacking or lifecycle ambiguity. Co-authored-by: Cursor <cursoragent@cursor.com>
2026-09-18 17:32:08 +08:00
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+## Why
+
+The group-buying page displays invented participant counts but no group can be opened, joined, completed, or expired. This capability requires a visible lifecycle tied to checkout intent and payment-time seat claims.
+
+## What Changes
+
+- Add shop-managed group-buying activities with localized presentation data, fixed minor-unit group price, required paid-member count, eligibility window, and a per-order item limit of quantity 1 for the activity SKU.
+- Add concrete group instances that can be opened, joined atomically up to capacity, succeed on reaching capacity, or expire.
+- Bind qualifying orders to a group instance at checkout, resolve group price server-side, and expose activity and open-group discovery to customers.
+- Claim paid membership at payment, not at cart checkout. Pending-payment cancellation restores SKU stock only and does not roll back paid seats. If the cancelled order opened a group that still has zero paid members, that empty group becomes `cancelled` and is no longer joinable. Paid groups that expire without filling remain identifiable for a later refund workflow.
+- Compose exclusively with other promotions: a SKU MUST NOT overlap an active flash sale; a shop order with group-buy intent SHALL reject a coupon. Restate coupon and flash-sale checkout/cancel clauses in this change's `order` spec.
+- Replace the display-only mall fixture and add shop-admin activity management.
+
+## Capabilities
+
+### New Capabilities
+- `group-buying`: Timed group-buying activities, group-instance lifecycle, joining, and public discovery.
+
+### Modified Capabilities
+- `order`: Checkout validates group participation and snapshots group-buy pricing.
+- `frontend-mall`: Group-buying pages use live activity and group state.
+- `frontend-shop-admin`: Shops manage their own group-buying activities.
+
+## Non-goals
+
+Refund execution, shipping holds, multi-product groups, platform campaigns, referral incentives, stacking coupons onto group-priced shop orders, and overlapping flash-sale pricing on the same SKU are excluded.
+
+## Impact
+
+Adds Rust marketing state-machine module/migrations, shared contract/locales, mall and shop-admin routes, and lifecycle/concurrency integration tests.