fix(edu-master): stop 20722d false alert on zeroed last_success gauge

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.

edu_master/k8s/alerts.yaml

@@ -11,9 +11,14 @@ spec:
       rules:
         # No successful webinar check for 5m (~2-3 missed 2-min checks).
         # Catches: playwright hangs/timeouts, version skew, site changes, hung job.
+        # The last_success > 0 guard is mandatory: checker.py initialises
+        # last_success to 0, so without it `time() - 0` equals the current epoch
+        # and humanizeDuration renders ~20722d on every pod restart. Keep the
+        # duration expression on the left so $value stays the real gap.
         - alert: WebinarCheckerNoSuccessfulCheck
           expr: |
-            (time() - webinar_check_last_success_timestamp_seconds > 300)
+            ((time() - webinar_check_last_success_timestamp_seconds) > 300)
+              and (webinar_check_last_success_timestamp_seconds > 0)
               and (webinar_check_last_run_timestamp_seconds > 0)
           for: 2m
           labels:
@@ -22,6 +27,20 @@ spec:
             summary: "Webinar checker has no successful check for 5m"
             description: "edu-master/webinar-checker: last successful webinar check was {{ $value | humanizeDuration }} ago. Checks are failing or hanging (see consecutive failures alert). Notifications about new webinars are NOT being sent."
 
+        # Checks are running but none has ever succeeded since pod start.
+        # Split out from the rule above so a zeroed gauge never feeds
+        # humanizeDuration.
+        - alert: WebinarCheckerNeverSucceeded
+          expr: |
+            (webinar_check_last_success_timestamp_seconds == 0)
+              and (webinar_check_last_run_timestamp_seconds > 0)
+          for: 10m
+          labels:
+            severity: critical
+          annotations:
+            summary: "Webinar checker has never completed a successful check"
+            description: 'edu-master/webinar-checker: checks have been running for 10m but not one has ever succeeded since the pod started, so every check is failing. Check pod logs (Loki: {namespace="edu-master", container="webinar-checker"}).'
+
         # Fast path: 3 consecutive failures (~6+ min at 2-min interval).
         - alert: WebinarCheckerConsecutiveFailures
           expr: |