Archive an eval prompt set

Archive (soft-delete) a prompt set. As part of the archive, Omni attempts to cancel every in-flight agentic job associated with the set; the returned cancelled_job_count reports how many were cancelled. The archive is committed before run cancellations start. Cancellation is best-effort — the database cancel is authoritative, but the Redis stop-signal that halts a running worker can lag. If the archive itself or a whole run-cancellation fails, the endpoint returns 500, but the prompt set is already archived. The call is idempotent — retrying drains any remaining runs.