test: add coverage for enqueueMany, stats, jobTimeout, and dead-letter

- Replace deleteOn:"failure" tests with dead-letter retention tests:
  verify dead-letters are kept under deleteOn:"success", and verify
  deleteOn:"always" still removes the row after exhausting retries
- Add dead-letter event test: emit "dead" exactly once after exhausting
  the default maxAttempts (5), verify deadLetteredAt is set
- Add jobTimeout test: worker exceeds timeout, signal is aborted, attempt
  fails with a timeout error and the job is dead-lettered
- Add stats() test: seed one job per state, assert mutually-exclusive counts
- Add enqueueMany() tests: bulk-insert three jobs, verify all are processed;
  empty-input fast-path returns 0
- Add purge() test: verify deadLetteredOnly removes only dead-letters,
  then verify full purge removes the remaining finished success, and the
  pending row is untouched