docs: add zigbee sensor battery monitoring findings

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

docs/plans/zigbee-sensor-battery-monitoring.md

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+# Zigbee Sensor Battery Monitoring
+
+## Problem
+
+Three Aqara Zigbee temperature sensors report `battery: 0` in their MQTT payload, making the `hass_sensor_battery_percent` Prometheus metric useless for battery monitoring on these devices.
+
+Affected sensors:
+- **Temp Living Room** (`0x54ef441000a54d3c`) — area: living_room
+- **Temp Office** (`0x54ef441000a547bd`) — area: office
+- **temp_server** — area: server_room
+
+The **Temp Bedroom** sensor (`0x00124b0025495463`) is a different model and reports battery correctly (69% at time of investigation).
+
+## Findings
+
+- All three sensors are actively reporting temperature, humidity, and pressure data — they are not dead.
+- The Zigbee2MQTT payload includes a `voltage` field (e.g., `2707` = 2.707V), which indicates healthy battery levels (~40-60% for a CR2032 coin cell).
+- CR2032 voltage reference: ~3.0V fresh, ~2.7V mid-life, ~2.1V dead.
+- The `voltage` field is not exposed as a Prometheus metric — it exists only in the MQTT payload.
+- This is a known firmware quirk with some Aqara sensors that always report 0% battery.
+
+## Possible Solutions
+
+### 1. Expose voltage as a Prometheus metric
+Enable the voltage sensor entities in Home Assistant (they may exist but be disabled by default). The HA Prometheus integration would then export them automatically.
+
+### 2. Calculate battery from voltage in Zigbee2MQTT
+Override the battery calculation using the voltage field. Approximate formula: `(voltage - 2100) / (3000 - 2100) * 100`.
+
+### 3. Alert on sensor staleness instead
+Create a Prometheus alert based on `hass_last_updated_time_seconds` going stale (e.g., no temperature update in 1 hour). This detects dead sensors regardless of battery reporting accuracy.