feat(observability): 📊 add backup failure alerting rules

VMRule alerts for forgejo-s3-backup and secrets-backup CronJobs:
- BackupCronJobNotScheduled (>26h since last run)
- BackupCronJobNeverScheduled (never ran)
- BackupJobFailed (job failed)
- BackupJobTooSlow (running >5min)

Ref: IPCEICIS-9313
Ref: IPCEICIS-2810

template/stacks/observability/victoria-k8s-stack/manifests/backup-alerts.yaml

@@ -0,0 +1,78 @@
+apiVersion: operator.victoriametrics.com/v1beta1
+kind: VMRule
+metadata:
+  name: backup-alerts
+  namespace: observability
+spec:
+  groups:
+    - name: backup-schedule-staleness
+      rules:
+        - alert: BackupCronJobNotScheduled
+          expr: |
+            time() - kube_cronjob_status_last_schedule_time{cronjob=~"forgejo-s3-backup|secrets-backup", namespace="gitea"}
+            > 26 * 3600
+          for: 5m
+          labels:
+            severity: critical
+            cronjob: "{{ $labels.cronjob }}"
+          annotations:
+            value: "{{ $value | humanizeDuration }}"
+            description: >-
+              CronJob {{ $labels.namespace }}/{{ $labels.cronjob }} has not been
+              scheduled for over 26 hours in cluster {{ $labels.cluster_environment }}.
+              Last schedule was {{ $value | humanizeDuration }} ago.
+            summary: "Backup CronJob {{ $labels.cronjob }} is stale"
+
+        - alert: BackupCronJobNeverScheduled
+          expr: |
+            kube_cronjob_status_last_schedule_time{cronjob=~"forgejo-s3-backup|secrets-backup", namespace="gitea"}
+            == 0
+          for: 30m
+          labels:
+            severity: critical
+            cronjob: "{{ $labels.cronjob }}"
+          annotations:
+            description: >-
+              CronJob {{ $labels.namespace }}/{{ $labels.cronjob }} has never been
+              scheduled in cluster {{ $labels.cluster_environment }}.
+            summary: "Backup CronJob {{ $labels.cronjob }} never ran"
+
+    - name: backup-job-failures
+      rules:
+        - alert: BackupJobFailed
+          expr: |
+            max by(cluster_environment, namespace, job_name) (
+              kube_job_status_failed{job_name=~"forgejo-s3-backup-.*|secrets-backup-.*", namespace="gitea"}
+            ) > 0
+          for: 30s
+          labels:
+            severity: critical
+            job_name: "{{ $labels.job_name }}"
+          annotations:
+            value: "{{ $value }}"
+            description: >-
+              Backup job {{ $labels.namespace }}/{{ $labels.job_name }} has
+              {{ $value }} failed pod(s) in cluster {{ $labels.cluster_environment }}.
+            summary: "Backup job {{ $labels.job_name }} failed"
+
+    - name: backup-job-duration
+      rules:
+        - alert: BackupJobTooSlow
+          expr: |
+            (
+              time() - kube_job_status_start_time{job_name=~"forgejo-s3-backup-.*|secrets-backup-.*", namespace="gitea"}
+            ) > 300
+            and
+            kube_job_status_active{job_name=~"forgejo-s3-backup-.*|secrets-backup-.*", namespace="gitea"} > 0
+          for: 1m
+          labels:
+            severity: major
+            job_name: "{{ $labels.job_name }}"
+          annotations:
+            value: "{{ $value | humanizeDuration }}"
+            description: >-
+              Backup job {{ $labels.namespace }}/{{ $labels.job_name }} has been
+              running for {{ $value | humanizeDuration }} (threshold: 5m)
+              in cluster {{ $labels.cluster_environment }}. This may indicate a
+              hung process or connectivity issue.
+            summary: "Backup job {{ $labels.job_name }} running too long"