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.

config/discovered_sensors.json.example

@@ -0,0 +1,32 @@
+[
+  {
+    "mac": "A4:C1:38:12:34:56",
+    "name": "ATC_123456",
+    "rssi": -65,
+    "first_seen": "2025-12-27T10:30:15",
+    "last_seen": "2025-12-27T10:35:42",
+    "sample_reading": {
+      "temperature": 21.5,
+      "humidity": 45,
+      "battery_percent": 87,
+      "battery_voltage": 2950
+    },
+    "status": "pending"
+  },
+  {
+    "mac": "A4:C1:38:AB:CD:EF",
+    "name": "ATC_ABCDEF",
+    "rssi": -72,
+    "first_seen": "2025-12-27T11:00:00",
+    "last_seen": "2025-12-27T11:10:00",
+    "sample_reading": {
+      "temperature": 19.8,
+      "humidity": 52,
+      "battery_percent": 65,
+      "battery_voltage": 2800
+    },
+    "status": "ignored",
+    "ignored_at": "2025-12-27T11:15:00",
+    "ignore_reason": "Test sensor, not needed"
+  }
+]