feat: add PrometheusRule alerts for all services

28 alert rules across 9 PrometheusRule files covering infrastructure
(Longhorn, cert-manager), data (PostgreSQL, OpenBao, OpenSearch),
storage (SeaweedFS), devtools (Gitea), identity (Hydra, Kratos),
media (LiveKit), and mesh (Linkerd golden signals for all services).

Severity routing: critical alerts fire to Matrix + email, warnings
to Matrix only (AlertManager config updated in separate commit).

base/data/openbao-alertrules.yaml

@@ -0,0 +1,28 @@
+apiVersion: monitoring.coreos.com/v1
+kind: PrometheusRule
+metadata:
+  name: openbao-alerts
+  namespace: data
+  labels:
+    role: alert-rules
+spec:
+  groups:
+    - name: openbao
+      rules:
+        - alert: VaultSealed
+          expr: vault_core_unsealed == 0
+          for: 1m
+          labels:
+            severity: critical
+          annotations:
+            summary: "OpenBao/Vault is sealed"
+            description: "OpenBao/Vault is sealed — automatic unseal may have failed"
+
+        - alert: VaultDown
+          expr: up{job=~".*openbao.*"} == 0
+          for: 2m
+          labels:
+            severity: critical
+          annotations:
+            summary: "OpenBao/Vault is down"
+            description: "OpenBao instance {{ $labels.namespace }}/{{ $labels.pod }} is down."