Request cancellation of a durable job

Cancellation is cooperative. Work already running may finish, while unstarted children are cancelled without consuming page quota.

Work already in flight is still charged. A capture is claimed by its worker within about a second of submission, so a cancellation for a single capture almost always arrives after the browser has started and that capture is billed as a completed one; the credit is not refunded. What cancellation saves is the unstarted remainder — for a batch or crawl, every child that had not yet been claimed. Read the response's counts rather than assuming a cancelled job cost nothing, and treat GET /credits as authoritative for what was actually spent.