Three fixes for the silent post-restart ticker stall:
1. _jobs_lock() bounds its cross-process flock: LOCK_NB polled against a
30s deadline instead of an unbounded LOCK_EX taken while holding the
process-wide RLock. On timeout it logs at ERROR and degrades to
in-process-only locking (the existing fallback path), so a sibling
process wedged while holding .jobs.lock can no longer freeze every
cron function - including the ticker's get_due_jobs() and thus the
heartbeat - forever with zero logging.
2. fire_claim/run_claim freshness checks are bounded on both sides
(0 <= age < ttl): a claim stamped in the future (clock/TZ skew across
a restart) was previously fresh forever, making the job permanently
unfireable and every manual run report 'already being fired'.
3. _execute_job_now distinguishes paused/disabled/missing jobs from a
genuinely held claim instead of mislabeling them all as 'already
being fired'.
