## 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, and requested coupon redemption when eligible. All amounts use the cart's SKU currencies converted into the buyer-chosen display currency at checkout time. For each cart line the server SHALL resolve an eligible active flash-sale item, enforce its remaining stock and customer limit, snapshot the final unit price and applied activity identity, and charge standard price for quantity not eligible for the activity. A SKU that is also eligible for group-buying in an overlapping window SHALL cause checkout to return 409. The request MAY select at most one owned coupon per generated shop order; the server SHALL validate 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. If any line on that shop order applied flash-sale pricing, the server SHALL reject a coupon for that shop with 409.

#### 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 or applicable flash-sale stock
- **THEN** the whole checkout returns 409 and no order is created and stock is unchanged

#### Scenario: active flash price applied
- **WHEN** checkout contains a SKU with an active eligible flash-sale item
- **THEN** the order item records the server-calculated flash-sale price and activity identity

#### Scenario: coupon applies to a shop without flash lines
- **WHEN** the customer selects an eligible shop coupon and that shop's generated order has no flash-priced lines
- **THEN** the order records that coupon and its server-calculated discount

#### Scenario: coupon rejected on a flash-priced shop order
- **WHEN** the customer selects a coupon for a shop whose generated order applied flash-sale pricing on any line
- **THEN** checkout returns 409 and creates no orders

### 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.

#### Scenario: cancel restores stock
- **WHEN** a customer cancels a pending_payment order
- **THEN** stock of each SKU and its applied flash-sale activity 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