Marks a paid purchase as handed over. The proof is the scanned QR token or the typed purchase reference, exactly one of the two. The token is verified against the order, an unpaid order is refused with 409 order_unpaid, and fulfilment moves to fulfilled under the order version precondition, recording fulfilled_at and the acting user.
Claiming an order that is already fulfilled writes nothing and answers 200 with first_claim false, so a second scan is safe and shows staff what happened the first time. A claim that races another write to the same order answers 400 version_conflict, the answer the fulfilment PATCH already gives.