Add MeshCore as a second backend alongside Meshtastic, enabling
simultaneous monitoring of both device types from a single dashboard.

- Modular backend architecture (MeshBackend ABC, MeshtasticBackend, MeshCoreBackend)
- Dual mode: monitor Meshtastic + MeshCore devices simultaneously
- Web UI: backend badges, filters, per-backend status indicators, maps for MeshCore positions
- Database: automatic migration adds backend column, backward compatible
- CLI: --backend, --mc-ble, --mc-serial, --mc-tcp arguments
- meshcore optional dependency via pip install meshconsole[meshcore]
- 70-test suite covering models, database, and MeshCore backend
- Zero breaking changes for existing Meshtastic-only users