Update a listing

Update any field on an item (price, title, description, status, etc.). This route does NOT accept an Idempotency-Key (check the parameters of an operation before sending one — this spec declares the header only on the six routes that honour it). Retry safety comes from the conflict check instead: on a timeout, re-read the item with GET and re-apply the change, and treat the "changed while you were editing" 409 below as your signal that a concurrent write landed first.