Get all results for an enumeration batch

All job results for a batch (GET /email/batch/{id}/results).

completed/failed are counted from the job rows in results, not read from mirrorball.batch's worker-maintained counters — the worker's attempts-exhausted sweep fails jobs without bumping those counters, so they undercount. Counting rows keeps the progress bar and the "batch done" signal (terminality of results) consistent by construction.