Delete Job

Purge a job's content artifacts (result, input file, rendered pages). The job's metadata (existence, state, timings) stays queryable at ``/jobs/{job_id}``; its result endpoint answers 410 afterwards. A job that is still running is cancelled first and recorded as failed, so the resume scan will not re-run it. Idempotent: deleting an already-purged job is a 204 no-op. Works with or without a configured ``result_ttl_s``.