feat: Add combined track functionality with repository and service layers

- Introduced CombinedTrackModel, CombinedTrackCreate, and CombinedTrackRepository for managing combined tracks.
- Implemented logic to create combined tracks based on existing tracks between two stations.
- Added methods to check for existing combined tracks and retrieve constituent track IDs.
- Enhanced TrackModel and TrackRepository to support OSM ID and track updates.
- Created migration scripts for adding combined tracks table and OSM ID to tracks.
- Updated services and API endpoints to handle combined track operations.
- Added tests for combined track creation, repository methods, and API interactions.

backend/app/repositories/__init__.py

@@ -2,6 +2,7 @@
 
 from backend.app.repositories.stations import StationRepository
 from backend.app.repositories.tracks import TrackRepository
+from backend.app.repositories.combined_tracks import CombinedTrackRepository
 from backend.app.repositories.train_schedules import TrainScheduleRepository
 from backend.app.repositories.trains import TrainRepository
 from backend.app.repositories.users import UserRepository
@@ -10,6 +11,7 @@ __all__ = [
     "StationRepository",
     "TrainScheduleRepository",
     "TrackRepository",
+    "CombinedTrackRepository",
     "TrainRepository",
     "UserRepository",
 ]