checker.py initialises last_success to 0, so right after a pod restart
`time() - last_success` equals the current epoch. The rule compared that
against 300, went firing instantly, and humanizeDuration rendered the raw
epoch delta as ~20722d. The last_run > 0 guard did not help because a run
happens long before the first success.
Guard the duration rule on last_success > 0, keeping the duration
expression on the left of `and` so $value stays the real gap, and add a
separate WebinarCheckerNeverSucceeded rule for the zeroed-gauge case so a
checker that has never succeeded is still caught.
Deploy workflow uses git-tracked manifests, DISABLED flag and kustomize overlays; add webinar-checker metrics with ServiceMonitor and alerts; upgrade shared postgres to 17 with statuspage DB and probes/resources.