1. Add missing `last_delivery_error` field initialization in `create_job()`.
`mark_job_run()` sets this field on line 596 but it was never initialized,
causing inconsistent job schemas between new and executed jobs.
2. Replace unnecessary `save_jobs()` call with a warning log when
`mark_job_run()` is called with a non-existent job_id. Previously the
function would silently write unchanged data to disk.
3. Add `cancel_futures=True` to the `finally` block in cron scheduler's
thread pool shutdown. The `except` path already passes this flag but
the normal exit path did not, leaving futures running after inactivity
timeout detection.
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