home-assistant: fix zigbee sensor battery reporting

WSDCGQ12LM sensors report battery: 0 due to firmware quirk. Override
battery calculation using voltage via homeassistant value_template.

Also adds zigbee_sensor_stale alert for detecting dead sensors regardless
of battery reporting accuracy (1 hour threshold).

Device configuration moved from external devices.yaml to inline NixOS
config for declarative management.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/monitoring/rules.yml

@@ -226,6 +226,14 @@ groups:
         annotations:
           summary: "Mosquitto not running on {{ $labels.instance }}"
           description: "Mosquitto has been down on {{ $labels.instance }} more than 5 minutes."
+      - alert: zigbee_sensor_stale
+        expr: (time() - hass_last_updated_time_seconds{entity=~"sensor\\.(0x[0-9a-f]+|temp_server)_temperature"}) > 3600
+        for: 5m
+        labels:
+          severity: warning
+        annotations:
+          summary: "Zigbee sensor {{ $labels.friendly_name }} is stale"
+          description: "Zigbee temperature sensor {{ $labels.entity }} has not reported data for over 1 hour. The sensor may have a dead battery or connectivity issues."
   - name: smartctl_rules
     rules:
       - alert: smart_critical_warning