Prefer UUIDs in the API (Phase 3 of the UUID migration)

- New /api/v1 endpoints: GET /soundtracks, /soundtracks/{uuid},
  /songs/{uuid} and /songs/{uuid}/file (streams the song, Range support,
  locked during sync)
- UUIDs added to every response with soundtracks or songs: queue info and
  list (SoundtrackUuid, SongUuid) and statistics (soundtrack_uuid,
  song_uuid). Existing fields stay for backward compatibility
- Play counts and broken-song removal use song UUIDs instead of
  soundtrack ID + song name, so songs with the same name are counted
  separately
- Statistics return an empty song list for soundtracks without songs
  instead of a single empty entry
- The legacy .id file cleanup stays until Phase 4 (#31)

Closes #29

internal/server/server.go

@@ -27,6 +27,7 @@ type Server struct {
 	db         Database
 	q          repository.Querier
 	music      *backend.MusicService
+	library    *backend.LibraryService
 	sync       *backend.SyncService
 	characters *backend.CharacterService
 	releases   *backend.ReleaseService
@@ -47,6 +48,7 @@ func New(cfg config.Config, q repository.Querier, database Database) (*Server, e
 		db:         database,
 		q:          q,
 		music:      backend.NewMusicService(q, cfg.SoundCheckPath),
+		library:    backend.NewLibraryService(q),
 		characters: characters,
 		releases:   backend.NewReleaseService(cfg.ReleasesURL),
 		stats:      backend.NewStatisticsService(q),