Cancel purchase

Cancel an entire purchase from the buyer side.

Use this endpoint when a user requests cancellation before fulfillment state makes cancellation invalid.

Rules:

• Purchase must belong to the authenticated user.
• Cancellation may fail when items are already shipped or otherwise non-cancellable.

Responses:

• Returns 204 when cancellation succeeds.
• Returns 400 when cancellation is not allowed by current order/item state.
• Returns 404 when purchase is missing or not owned.