Replace all SongGroup/SongArrangementGroup model usages with Label/SongArrangementLabel
after the schema migration to global labels. Updates 12 app files and 11 test files:
- SongService: createDefaultGroups now finds-or-creates global Labels by name
- ArrangementController: validates label_id (labels are global, no song-ownership)
- ProImportService: imports groups as Labels (firstOrCreate by name); does not
overwrite existing label colors per spec
- ProExportService/SongPdfController/TranslationService/etc: traverse via
arrangements -> arrangementLabels -> label -> songSlides chain
- All test factories and assertions adapted to label-based schema