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/Dockerfile

@@ -25,7 +25,7 @@ EXPOSE 8000
 
 # Initialize database with demo data if INIT_DEMO_DB is set
 CMD ["sh", "-c", "\
-    export PYTHONPATH=/app && \
+    export PYTHONPATH=/app/backend && \
     echo 'Waiting for database...' && \
     while ! pg_isready -h db -p 5432 -U railgame >/dev/null 2>&1; do sleep 1; done && \
     echo 'Database is ready!' && \