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>
