Cancel order

Cancel an order from the seller side.

Use this endpoint when a seller cannot fulfill an order and needs to trigger cancellation logic.

Rules:

• Seller must own the order.
• Cancellation is blocked for invalid states (for example already shipped or non-cancellable items).

Responses:

• Returns 204 when cancellation succeeds.
• Returns 400 for non-cancellable orders.
• Returns 404 when the order is not found for the seller.