[monitoring] Add GPUNodeUnschedulable alert — fires when GPU node is cordoned

After k8s-node1 was silently cordoned and broke Frigate camera streams,
existing alerts (NvidiaExporterDown, PodUnschedulable) didn't catch the
root cause proactively. This alert fires within 5m of the GPU node being
cordoned, before any pod restart attempts to schedule and fails.

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

stacks/monitoring/modules/monitoring/prometheus_chart_values.tpl

@@ -750,6 +750,14 @@ serverFiles:
               severity: critical
             annotations:
               summary: "NVIDIA GPU exporter is down - no GPU metrics available"
+          - alert: GPUNodeUnschedulable
+            expr: kube_node_spec_unschedulable{node="k8s-node1"} == 1
+            for: 5m
+            labels:
+              severity: critical
+              subsystem: gpu
+            annotations:
+              summary: "GPU node {{ $labels.node }} is cordoned — Frigate and GPU workloads cannot schedule"
       - name: Power
         rules:
           - alert: OnBattery