Implement OwnTracks MQTT provider

[dcondrey]

Difficulty: Complex

Implement an OwnTracks provider using the `LocationProvider` ABC from `providers.py`.

OwnTracks publishes location updates over MQTT. This provider would:

1. Subscribe to an MQTT topic (e.g. `owntracks/+/+`)
2. Parse the OwnTracks JSON payload
3. Convert to `PersonLocation` objects
4. Run as a background listener alongside the poll loop

Considerations:

• Needs an MQTT client library (`paho-mqtt`)
• OwnTracks is push-based (not poll-based) — the provider interface may need a `subscribe(callback)` method in addition to `get_locations()`
• Config: MQTT broker URL, username, password, topic prefix
• TLS support for secure MQTT connections