feat(monitoring): align storage to live pvc, add loki datasource and alerts

Match grafana/alertmanager storageClass to live PVCs to avoid immutable-field failures. Add Loki datasource and LokiDown/AlloyDown alerts.

prometheus-stack/k8s/alerts.yaml

@@ -66,3 +66,21 @@ spec:
           annotations:
             summary: "Node memory pressure"
             description: "Node {{ $labels.instance }} has used more than 90% of memory for 15 minutes."
+
+        - alert: LokiDown
+          expr: kube_statefulset_status_replicas_unavailable{statefulset="loki"} > 0 or kube_deployment_status_replicas_unavailable{deployment="loki-gateway"} > 0
+          for: 10m
+          labels:
+            severity: warning
+          annotations:
+            summary: "Loki is down"
+            description: "Loki in namespace {{ $labels.namespace }} has unavailable replicas for more than 10 minutes. Logs are not queryable."
+
+        - alert: AlloyDown
+          expr: kube_daemonset_status_number_unavailable{daemonset="alloy"} > 0
+          for: 10m
+          labels:
+            severity: warning
+          annotations:
+            summary: "Alloy is down"
+            description: "Alloy DaemonSet in namespace {{ $labels.namespace }} has {{ $value }} unavailable pods for more than 10 minutes. Pod logs are not being shipped to Loki."