fix(observability): 🐛 add missing manifests to instance stacks

- backup-alerts.yaml → observability.buildth.ing victoria-k8s-stack
- forgejo-scrape.yaml → dev.t09.de vm-client-stack

otc/dev.t09.de/stacks/observability-client/vm-client-stack/manifests/forgejo-scrape.yaml

@@ -0,0 +1,15 @@
+apiVersion: operator.victoriametrics.com/v1beta1
+kind: VMServiceScrape
+metadata:
+  name: forgejo
+  namespace: observability
+spec:
+  namespaceSelector:
+    matchNames:
+      - gitea
+  selector:
+    matchLabels:
+      app.kubernetes.io/name: forgejo
+  endpoints:
+    - port: http
+      path: /metrics