Cancel a job

Review #82 / D45 — cooperative job cancellation.

Semantics:

• status=queued → flip to "cancelled" immediately; worker will skip it when picked up.
• status=running → set cancel_requested=True; long-running compute steps that check the flag will exit early. Status is not flipped here because the worker owns the lifecycle.
• status=complete | failed | cancelled → 409 Conflict (terminal state). Idempotent re-cancel is intentionally rejected so the UI can distinguish "I cancelled this" from "this already finished".

Workers can read the flag via jobs.get(job_id, {}).get('cancel_requested') at checkpoints.