feat: wake poll loop on enqueue for low-latency job pickup

Add #wake() / #waitForPoll() so a job enqueued during the idle
pollInterval sleep is picked up immediately rather than after the full
interval. A latch (#pendingWake) handles the race where enqueue fires
before the loop enters the next idle wait.

Also tighten the saturation back-off: when all concurrency slots are
occupied the loop now waits jobInterval (not the full pollInterval) so
the next free slot is picked up promptly.

Bump DEFAULT_STALE_TIMEOUT from 30 min to 6 hours with a note in the
JSDoc: the lease ceiling should exceed the longest expected job duration
to avoid reclaiming a still-running worker.