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>

openspec/changes/add-group-buying/specs/order/spec.md

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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, active flash-sale resolution when no group-buy intent applies to the SKU, requested group-buying intent validation, and requested coupon redemption when eligible. All amounts use the cart's SKU currencies converted into the buyer-chosen display currency at checkout time. A group-buy intent SHALL contain exactly one active activity SKU at quantity 1 and either an open group identifier or a request to open a group; the server SHALL snapshot the group price and group identity on the resulting pending-payment order. Group intent SHALL return 409 when that SKU is also eligible for an overlapping flash sale. For each remaining cart line without group intent the server SHALL resolve an eligible active flash-sale item as specified by flash-sales, snapshot the final unit price and activity identity, and charge standard price for ineligible quantity. The request MAY select at most one owned coupon per generated shop order; the server SHALL persist `coupon_id`, `discount_minor`, and post-discount `total_minor` only after ownership, shop, status, window, and threshold validation. The client SHALL NOT provide a discount amount. The server SHALL reject a coupon (409) for any shop order that applied flash-sale pricing or carries group-buy intent.
+
+#### Scenario: two shops → two orders
+- **WHEN** the cart contains SKUs from shops A and B without group-buy intent
+- **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 or applicable flash-sale stock
+- **THEN** the whole checkout returns 409 and no order is created and stock is unchanged
+
+#### Scenario: open a group at checkout
+- **WHEN** a customer checks out one eligible group-buying SKU at quantity 1 with open-group intent
+- **THEN** a pending-payment order and an open group identity are created with the server-resolved group price
+
+#### Scenario: coupon rejected on a group shop order
+- **WHEN** the customer selects a coupon for the shop that carries group-buy intent
+- **THEN** checkout returns 409 and creates no orders
+
+#### Scenario: coupon applies to a shop without activity prices
+- **WHEN** the customer selects an eligible shop coupon and that shop's generated order has no flash-priced lines and no group-buy intent
+- **THEN** the 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 SKU stock, any reserved flash-sale activity stock consumed by the order, and a redeemed coupon to claimed status when the order has one. Pending-payment cancellation SHALL NOT change group paid-member counts. When the cancelled order opened the group and paid member count is zero, the group SHALL become cancelled. Payment of an order with group-buying intent SHALL also validate and atomically claim an open group seat; full, expired, or cancelled groups SHALL return 409 and leave the order pending_payment.
+
+#### Scenario: group payment wins final seat
+- **WHEN** payment fills the final available group seat
+- **THEN** the payment succeeds and the group state becomes successful
+
+#### Scenario: cancel restores stock without group rollback
+- **WHEN** a customer cancels a pending_payment group-intent order
+- **THEN** SKU stock increases by the ordered qty, status is cancelled, and no paid group membership is removed
+
+#### Scenario: unpaid opener cancel closes an empty group
+- **WHEN** the opening pending_payment order is cancelled and the group has zero paid members
+- **THEN** the group status becomes cancelled and a later payment against that group returns 409
+
+#### 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