Implement sensor auto-discovery feature

New Features:
- Automatic discovery of unknown Bluetooth sensors
- Discovery manager tracks pending/approved/ignored sensors
- ntfy notifications when new sensors found (optional)
- Interactive CLI tool: sensorpajen-approve-sensors
- Automatic config reload every 15 minutes (no restart needed)

Files Added:
- src/sensorpajen/discovery_manager.py: Sensor discovery management
- src/sensorpajen/approve_sensors.py: Interactive approval CLI
- config/discovered_sensors.json.example: Example discovery file

Files Modified:
- src/sensorpajen/config.py: Added ntfy and discovery config
- src/sensorpajen/main.py: Added discovery manager and config reload
- src/sensorpajen/sensor_reader.py: Added discovery on unknown sensors
- config/sensorpajen.env.example: Added ntfy and reload settings
- pyproject.toml: Added approve-sensors CLI command

Configuration:
- NTFY_ENABLED, NTFY_URL, NTFY_TOPIC, NTFY_TOKEN
- DISCOVERED_SENSORS_FILE, CONFIG_RELOAD_INTERVAL
- Pre-filled comments with sensor metadata

See TASKS.md for complete feature specification.

TASKS.md

@@ -86,7 +86,7 @@ Implement **automatic sensor discovery** with a **user approval workflow** that:
   * If `NTFY_ENABLED=false`, skip notifications
   * If ntfy is unreachable, log error and continue
   * Discovery and approval must work even if ntfy fails
-
+* The user must only be notified once per discovered sensor
 ---
 
 ### 4. User Approval Script