Get enumeration job status

Poll a single job -- owner-scoped exactly like every other read here.

enumerate_email now enqueues every job as a one-job batch (see its docstring), so this poll also drives that batch's finalization/billing: it looks up the owning batch (also owner-scoped, so this adds no new IDOR surface), fetches its results, and syncs/finalizes through the same _sync_batch_task helper get_batch_results uses below -- no separate copy of the billable-status decision or the CAS. A job with no batch (none found, or the batch row aged past its purge window) just skips straight to returning the job, same as before this change.