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/storage/kustomization.yaml

@@ -12,3 +12,5 @@ resources:
   - seaweedfs-filer-pvc.yaml
   - vault-secrets.yaml
   - seaweedfs-remote-sync.yaml
+  - seaweedfs-servicemonitor.yaml
+  - seaweedfs-alertrules.yaml

base/storage/seaweedfs-alertrules.yaml

@@ -0,0 +1,46 @@
+apiVersion: monitoring.coreos.com/v1
+kind: PrometheusRule
+metadata:
+  name: seaweedfs-alerts
+  namespace: storage
+  labels:
+    role: alert-rules
+spec:
+  groups:
+    - name: seaweedfs
+      rules:
+        - alert: SeaweedFSMasterDown
+          expr: up{job=~".*seaweedfs.*", app="seaweedfs-master"} == 0
+          for: 2m
+          labels:
+            severity: critical
+          annotations:
+            summary: "SeaweedFS master is down"
+            description: "SeaweedFS master {{ $labels.namespace }}/{{ $labels.pod }} is down."
+
+        - alert: SeaweedFSVolumeDown
+          expr: up{job=~".*seaweedfs.*", app="seaweedfs-volume"} == 0
+          for: 2m
+          labels:
+            severity: critical
+          annotations:
+            summary: "SeaweedFS volume server is down"
+            description: "SeaweedFS volume server {{ $labels.namespace }}/{{ $labels.pod }} is down."
+
+        - alert: SeaweedFSFilerDown
+          expr: up{job=~".*seaweedfs.*", app="seaweedfs-filer"} == 0
+          for: 2m
+          labels:
+            severity: critical
+          annotations:
+            summary: "SeaweedFS filer is down"
+            description: "SeaweedFS filer {{ $labels.namespace }}/{{ $labels.pod }} is down."
+
+        - alert: SeaweedFSDiskLow
+          expr: (seaweedfs_disk_free_bytes / (seaweedfs_disk_free_bytes + seaweedfs_disk_used_bytes)) < 0.15
+          for: 5m
+          labels:
+            severity: warning
+          annotations:
+            summary: "SeaweedFS disk space low"
+            description: "SeaweedFS disk on {{ $labels.instance }} has less than 15% free space."