Update purchase order

Apply a buyer-side update to a specific order inside a purchase.

Use this endpoint for post-purchase item actions such as delivery confirmation or opening a dispute.

Rules:

• Purchase must belong to the authenticated user.
• Target orderId must exist within that purchase.
• Cancellation is intentionally excluded from this endpoint and handled by the dedicated cancel route.

Responses:

• Returns 204 on success.
• Returns 404 for missing purchase or order.