v-m: alerts: Enhance Frigate unavailable alert

If Frigate is running but not connected to the MQTT broker, the
`sensor.frigate_status` entity will be available, but the
`update.frigate_server` entity will not.

victoria-metrics/alerts.yml

@@ -126,4 +126,4 @@ groups:
   rules:
   - alert: Frigate is Unavailable
     expr:
-      homeassistant_entity_available{entity="sensor.frigate_status"} != 1
+      homeassistant_entity_available{entity=~".*frigate_(server|status)"} != 1