Follow-ups on the #87261 salvage:
- cron/jobs.py: the persisted-error re-arm now respects schedule legality.
Re-arming to `now` fired CRON jobs at times their expression excludes —
a weekday-only 9am job whose Friday run errored would fire on SATURDAY
(croniter measures a 24h cadence on Saturday, so 27h > cadence+grace and
the guard tripped). Cron jobs re-arm to compute_next_run(schedule, now)
— the next LEGAL occurrence — and only when that actually moves
next_run_at earlier; interval jobs (the 2026-08-14 incident class) keep
the immediate now re-arm, which is always legal for intervals.
- cron/jobs.py: cache _schedule_cadence_seconds' croniter measurement per
expr (mirrors scheduler.py's _cron_interval_cache) — it runs inside
_jobs_lock on every tick for every stale-errored job.
- tests/cron/test_persisted_error_rearm_legality.py (new): weekday job
errored Friday re-arms to Monday (not Saturday), correctly-parked cron
value untouched, interval job still due immediately.
kshitij