- Add backupDatabase() for timestamped .db file backup before migration
- Add migration6MultiLibrary() with all 14 steps: FK OFF, create libraries table,
insert default library from TOML config, add library_id to audio_files, rebuild
playlist_tracks with SET NULL FK and 6 phantom columns, backfill phantom metadata,
recreate track_metadata VIEW with library_id, FK ON, clean TOML config
- Add readLibraryDirFromTOML() and removeLibraryDirFromTOML() helpers
- Update runMigrations signature to accept dbPath for backup
- Add sentinel library row in NewTestDB for FK constraint satisfaction
yonlu
1179f56c36