feat: add telemetry collection for companion and repeater nodes (#24)

Add environmental telemetry collection (temperature, humidity, barometric
pressure, voltage) from both the repeater node (over LoRa) and companion
node (local serial). Telemetry is stored in the same EAV metrics table
with `telemetry.` prefix.

Key changes:
- Add TELEMETRY_ENABLED feature flag (defaults to OFF)
- Add telemetry-specific timeout/retry settings
- Create shared telemetry.py module with extract_lpp_from_payload()
  and extract_telemetry_metrics() helpers
- Handle MeshCore API dict payload format: {'pubkey_pre': '...', 'lpp': [...]}
- Repeater: store status metrics BEFORE attempting telemetry (LoRa reliability)
- Companion: merge telemetry into single DB write (serial is reliable)
- Telemetry failures do NOT affect circuit breaker state

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

docs/firmware-responses.md

@@ -102,6 +102,84 @@ Returns a single dict with all status fields.
 
 ---
 
+## Telemetry Data
+
+Environmental telemetry is requested via `req_telemetry_sync(contact)` and returns
+Cayenne LPP formatted sensor data. This requires `TELEMETRY_ENABLED=1` and a sensor
+board attached to the repeater.
+
+### Payload Format
+
+Both `req_telemetry_sync()` and `get_self_telemetry()` return a dict containing the
+LPP data list and a public key prefix:
+
+```python
+{
+    'pubkey_pre': 'a5c14f5244d6',
+    'lpp': [
+        {'channel': 0, 'type': 'temperature', 'value': 23.5},
+        {'channel': 0, 'type': 'humidity', 'value': 45.2},
+    ]
+}
+```
+
+The `extract_lpp_from_payload()` helper in `src/meshmon/telemetry.py` handles
+extracting the `lpp` list from this wrapper format.
+
+### `req_telemetry_sync(contact)`
+
+Returns sensor readings from a remote node in Cayenne LPP format:
+
+```python
+[
+    {'channel': 0, 'type': 'temperature', 'value': 23.5},
+    {'channel': 0, 'type': 'humidity', 'value': 45.2},
+    {'channel': 0, 'type': 'barometer', 'value': 1013.25},
+    {'channel': 1, 'type': 'gps', 'value': {'latitude': 51.5, 'longitude': -0.1, 'altitude': 10}},
+]
+```
+
+**Common sensor types:**
+
+| Type | Unit | Description |
+|------|------|-------------|
+| `temperature` | Celsius | Temperature reading |
+| `humidity` | % | Relative humidity |
+| `barometer` | hPa/mbar | Barometric pressure |
+| `voltage` | V | Voltage reading |
+| `gps` | compound | GPS with `latitude`, `longitude`, `altitude` |
+
+**Stored as:**
+- `telemetry.temperature.0` - Temperature on channel 0
+- `telemetry.humidity.0` - Humidity on channel 0
+- `telemetry.gps.1.latitude` - GPS latitude on channel 1
+
+**Notes:**
+- Requires environmental sensor board (BME280, BME680, etc.) on repeater
+- Channel number distinguishes multiple sensors of the same type
+- Not all repeaters have environmental sensors attached
+- Telemetry collection does not affect circuit breaker state
+- Telemetry failures are logged as warnings and do not block status collection
+
+### `get_self_telemetry()`
+
+Returns self telemetry from the companion node's attached sensors.
+Same Cayenne LPP format as `req_telemetry_sync()`.
+
+```python
+[
+    {'channel': 0, 'type': 'temperature', 'value': 23.5},
+    {'channel': 0, 'type': 'humidity', 'value': 45.2},
+]
+```
+
+**Notes:**
+- Requires environmental sensor board attached to companion
+- Returns empty list if no sensors attached
+- Uses same format as repeater telemetry
+
+---
+
 ## Derived Metrics
 
 These are computed at query time, not stored: