feat(observability): 🎉 Add new VMRule for forgejo alerts

Introduces a new alerting rule for monitoring the forgejo application in the observability namespace. This rule triggers alerts when the forgejo server is down, helping to improve system reliability and response times.

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

@@ -0,0 +1,18 @@
+apiVersion: operator.victoriametrics.com/v1beta1
+kind: VMRule
+metadata:
+  name: forgejo-alerts
+  namespace: observability
+spec:
+  groups:
+    - name: forgejo
+      rules:
+        - alert: forgejo down
+          expr: sum by(cluster_environment) (up{pod=~"forgejo-server-.*"}) < 1
+          for: 30s
+          labels:
+            severity: major
+            job:  "{{ $labels.job }}"
+          annotations:
+            value: "{{ $value }}"
+            description: 'forgejo is down in cluster environment {{ $labels.cluster_environment }}'