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>

meshcore.conf.example

@@ -113,6 +113,23 @@ RADIO_CODING_RATE=CR8
 # REMOTE_CB_FAILS=6
 # REMOTE_CB_COOLDOWN_S=3600
 
+# =============================================================================
+# Telemetry Collection (Environmental Sensors)
+# =============================================================================
+# Enable telemetry collection from repeater's environmental sensors
+# (temperature, humidity, barometric pressure, etc.)
+# Requires sensor board attached to repeater (e.g., BME280, BME680)
+# Default: 0 (disabled)
+# TELEMETRY_ENABLED=1
+
+# Telemetry-specific timeout and retry settings
+# Defaults match status settings. Separate config allows tuning if telemetry
+# proves problematic (e.g., firmware doesn't support it, sensor board missing).
+# You can reduce these if telemetry collection is causing issues.
+# TELEMETRY_TIMEOUT_S=10
+# TELEMETRY_RETRY_ATTEMPTS=2
+# TELEMETRY_RETRY_BACKOFF_S=4
+
 # =============================================================================
 # Paths (Native installation only)
 # =============================================================================