iTunes/Apple Music sync

[timusus]

…nc (Phase 1)

Implements Phase 1 of iTunes/Apple Music sync functionality as requested in issue #107.

Database Changes:

• Add rating column (0-5 stars) to songs table
• Create database migration MIGRATION_40_41
• Update MediaDatabase version to 41

Rating Support:

• Add rating field to Song model and SongData entity
• Implement SongRepository.setRating() for updating ratings
• Add updateRating() method to SongDataDao
• Display ratings in Song Info dialog with star visualization

MediaStore Integration:

• Import existing ratings from Android MediaStore
• Convert MediaStore ratings (0-100) to 0-5 star scale
• Automatic import during media scanning

Playlist Export:

• Create PlaylistExporter utility class
• Support M3U/M3U8 format export
• Generate iTunes-compatible playlist files
• Support absolute and relative file paths

Documentation:

• Add ITUNES_SYNC_IMPLEMENTATION.md with complete implementation details
• Document future phases for full iTunes sync support
• Include API reference and testing recommendations

This implementation provides the foundation for iTunes/Apple Music synchronization. Future phases will add rating edit UI, playlist export UI, and desktop sync application.

Addresses: #107