fix(monitoring): use correct iSCSI storage classes for Prometheus and Alertmanager

local-path does not exist on this k3s cluster. The Prometheus Operator
was stuck in a reconcile error loop, blocking all PodMonitor updates.

Prometheus: merlin-local-iscsi-prometheus (30Gi pre-existing PV)
Alertmanager: merlin-local-iscsi-alertmanager (5Gi pre-existing PV)
Grafana: remains local-path to match immutable StatefulSet VolumeClaimTemplate;
         actual PVC is already bound to merlin-local-iscsi-grafana PV.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

monitoring/kps-values-k3s.yaml

@@ -29,7 +29,7 @@ alertmanager:
           resources:
             requests:
               storage: 5Gi
-          storageClassName: local-path
+          storageClassName: merlin-local-iscsi-alertmanager
 
 grafana:
   enabled: true
@@ -48,6 +48,8 @@ grafana:
     - ReadWriteOnce
     enabled: true
     size: 10Gi
+    # StatefulSet VolumeClaimTemplate is immutable; must stay "local-path" to match the
+    # existing StatefulSet spec. Actual PVC binds to merlin-local-iscsi-grafana PV.
     storageClassName: local-path
     type: sts
 
@@ -123,7 +125,7 @@ prometheus:
           resources:
             requests:
               storage: 30Gi
-          storageClassName: local-path
+          storageClassName: merlin-local-iscsi-prometheus
 
 prometheus-node-exporter:
   affinity: