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/postgres-alertrules.yaml

@@ -0,0 +1,37 @@
+apiVersion: monitoring.coreos.com/v1
+kind: PrometheusRule
+metadata:
+  name: postgres-alerts
+  namespace: data
+  labels:
+    role: alert-rules
+spec:
+  groups:
+    - name: postgres
+      rules:
+        - alert: PostgresDown
+          expr: cnpg_collector_up == 0
+          for: 2m
+          labels:
+            severity: critical
+          annotations:
+            summary: "PostgreSQL instance is down"
+            description: "CNPG collector reports {{ $labels.namespace }}/{{ $labels.pod }} is down."
+
+        - alert: PostgresDatabaseSizeLarge
+          expr: cnpg_pg_database_size_bytes > 7e9
+          for: 5m
+          labels:
+            severity: warning
+          annotations:
+            summary: "PostgreSQL database size is large"
+            description: "Database {{ $labels.datname }} is {{ $value | humanize1024 }} (PVC limit 10Gi)"
+
+        - alert: PostgresHighConnections
+          expr: sum by (pod) (cnpg_pg_stat_activity_count) > 80
+          for: 5m
+          labels:
+            severity: warning
+          annotations:
+            summary: "PostgreSQL connection count is high"
+            description: "Pod {{ $labels.pod }} has {{ $value }} active connections."