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/opensearch-alertrules.yaml

@@ -0,0 +1,37 @@
+apiVersion: monitoring.coreos.com/v1
+kind: PrometheusRule
+metadata:
+  name: opensearch-alerts
+  namespace: data
+  labels:
+    role: alert-rules
+spec:
+  groups:
+    - name: opensearch
+      rules:
+        - alert: OpenSearchClusterRed
+          expr: opensearch_cluster_health_status{color="red"} == 1
+          for: 2m
+          labels:
+            severity: critical
+          annotations:
+            summary: "OpenSearch cluster health is RED"
+            description: "OpenSearch cluster {{ $labels.cluster }} health status is red."
+
+        - alert: OpenSearchClusterYellow
+          expr: opensearch_cluster_health_status{color="yellow"} == 1
+          for: 10m
+          labels:
+            severity: warning
+          annotations:
+            summary: "OpenSearch cluster health is YELLOW"
+            description: "OpenSearch cluster {{ $labels.cluster }} health status is yellow."
+
+        - alert: OpenSearchHeapHigh
+          expr: (opensearch_jvm_mem_heap_used_bytes / opensearch_jvm_mem_heap_max_bytes) > 0.85
+          for: 5m
+          labels:
+            severity: warning
+          annotations:
+            summary: "OpenSearch JVM heap usage is high"
+            description: "OpenSearch node {{ $labels.node }} in {{ $labels.namespace }} heap usage is above 85%."