Drives a job to the terminal canceled state on request, freeing its
active-job slot immediately. Takes no request body.

Cancel does not delete the job: the job record and any results already
produced are preserved for the normal retention window, the same as a
completed or failed job. Items stop being processed and keep the state
they held at cancellation, so a canceled job's counts may show
unfinished items that never resolve.

Cancel is idempotent: canceling an already-canceled job returns 200
with the job. Canceling a job that has already completed or failed is
a 409.