feat: add enqueueMany, stats, jobTimeout, and fix dead-letter retention

enqueueMany(payloads, opts): bulk-inserts many plain jobs in a single
createMany (one round-trip); wakes the poll loop on return.

stats(): returns mutually-exclusive counts by state (pending, scheduled,
processing, completed, dead) for monitoring/alerting on backlog depth and
dead-letter accumulation.

jobTimeout option (non-transactional mode only): per-job wall-clock cap.
On expiry the job's signal is aborted (cooperative cancellation) and the
attempt fails — retried or dead-lettered as usual. Backed by a dedicated
per-job AbortController (#createJobAbort) that is linked to the queue-wide
controller so stop() and per-job timeout both abort the signal, with
cleanup() to detach the link once the job settles.

Fix dead-letter retention: the "failure" value is removed from the deleteOn
enum — dead-letters are now retained by default under "never" and "success"
so the DLQ stays inspectable; only "always" (an explicit opt-out) also
deletes dead-letters. The internal checks are updated accordingly.