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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+## MODIFIED Requirements
+
+### Requirement: Checkout splits by shop
+`POST /api/orders/checkout` SHALL create one order per distinct shop in the cart, in a single database transaction: stock decrement, order + item insert with price snapshots, cart clear, and requested coupon redemption. All amounts use the cart's SKU currencies converted into the buyer-chosen display currency at checkout time. The request MAY select at most one owned coupon per generated shop order; the server SHALL validate its ownership, shop, status, validity window, threshold, and converted amount, then persist `coupon_id`, `discount_minor`, and the post-discount `total_minor`. The client SHALL NOT provide a discount amount. This requirement does not define activity prices; a later flash-sale or group-buying change that rewrites checkout SHALL restate these coupon clauses and SHALL reject a coupon on any shop order that applied an activity price.
+
+#### Scenario: two shops → two orders
+- **WHEN** the cart contains SKUs from shops A and B
+- **THEN** two orders are created, each with only its shop's items, and the cart is empty
+
+#### Scenario: insufficient stock
+- **WHEN** any line's qty exceeds SKU stock
+- **THEN** the whole checkout returns 409 and no order is created and stock is unchanged
+
+#### Scenario: coupon applies to one matching shop
+- **WHEN** the customer selects an eligible coupon issued by shop A for a cart containing shops A and B
+- **THEN** only shop A's order records that coupon and its server-calculated discount
+
+### Requirement: Order lifecycle
+Status transitions SHALL be: pending_payment → paid → fulfilling → shipped → completed; cancellable only from pending_payment, which MUST restore stock and restore a redeemed coupon to claimed status when the order has one.
+
+#### Scenario: cancel restores stock
+- **WHEN** a customer cancels a pending_payment order
+- **THEN** stock of each SKU increases by the ordered qty and status is cancelled
+
+#### Scenario: cancel restores coupon
+- **WHEN** a customer cancels a pending_payment order with a redeemed coupon
+- **THEN** the coupon is detached from the order and becomes claimed
+
+#### Scenario: illegal transition rejected
+- **WHEN** cancelling a paid order via the customer endpoint
+- **THEN** the API returns 409