nightly cron stopped running 23 days ago and nothing told me Cron

Check the Actions tab for a banner saying the workflow was disabled. Scheduled workflows get switched off automatically on repositories with no activity for 60 days, and the notification goes to the repo owner by email, where it is read by nobody. I would put money on that being your 23 days.

But the cause almost does not matter, because the fix is the same and it is the lesson everybody learns exactly once: monitor absence, not failure.

The job pings a heartbeat URL as its last successful statement. The monitor alerts when the ping does not arrive inside a window. Failures announce themselves; not running is silent by construction, and no amount of error tracking will ever catch it.

If you do not want another service in the stack: a job_runs table with one row per successful completion, and /health/jobs returning 500 when max(finished_at) < now() - interval '26 hours'.

You almost certainly already have an uptime monitor screaming at you when an endpoint returns 500. Now it screams about cron too, and you added one table and one route.

Alert on the output, not the process.

If the digest is the product, count digests sent per day and alert when the count drops below a floor. Process monitoring tells you a thing executed. Output monitoring tells you a customer received something. They fail in different ways - I have had a job run perfectly every night while a mail provider silently dropped everything to one domain.

Add a second workflow that checks whether the first one ran and opens an issue if it did not.

Also move it off Actions. Scheduled workflows are for maintenance chores where nobody notices a missed night. Anything customer-facing wants a scheduler that is part of your app's platform, where a missed run is visible in the same place as everything else you look at.