[ci skip] Add HomeAssistantDown alert for ha-sofia

Fires after 5m if the haos Prometheus scrape target is unreachable.
Covers the HTTP API endpoint which shares the same process as the
WebSocket API used by the mobile app.

modules/kubernetes/monitoring/prometheus_chart_values.tpl

@@ -316,6 +316,13 @@ serverFiles:
               summary: "Job {{ $labels.namespace }}/{{ $labels.job_name }}: {{ $value | printf \"%.0f\" }} failure(s)"
       - name: Infrastructure Health
         rules:
+          - alert: HomeAssistantDown
+            expr: up{job="haos"} == 0
+            for: 5m
+            labels:
+              severity: page
+            annotations:
+              summary: "Home Assistant down: {{ $labels.instance }}"
           - alert: CoreDNSErrors
             expr: rate(coredns_dns_responses_total{rcode="SERVFAIL"}[5m]) > 1
             for: 10m