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/routes.go

@@ -38,6 +38,7 @@ type handlers struct {
 	download   *DownloadHandler
 	sync       *SyncHandler
 	music      *MusicHandler
+	library    *LibraryHandler
 	statistics *StatisticsHandler
 	token      *TokenHandler
 }
@@ -50,6 +51,7 @@ func (s *Server) handlers() handlers {
 		download:   NewDownloadHandler(s.releases),
 		sync:       NewSyncHandler(s.sync),
 		music:      NewMusicHandler(s.music),
+		library:    NewLibraryHandler(s.library),
 		statistics: NewStatisticsHandler(s.stats),
 		token:      NewTokenHandler(s.q, s.cfg.APIPassword, s.cfg.DisableSessionAuth),
 	}
@@ -103,6 +105,11 @@ func (s *Server) RegisterRoutes() http.Handler {
 	registerCommon(protected, h, syncLock)
 	protected.GET("/character/list", h.character.GetCharacterList)
 
+	protected.GET("/soundtracks", h.library.GetSoundtracks)
+	protected.GET("/soundtracks/:uuid", h.library.GetSoundtrack)
+	protected.GET("/songs/:uuid", h.library.GetSong)
+	protected.GET("/songs/:uuid/file", h.library.GetSongFile, syncLock)
+
 	stats := protected.Group("/statistics")
 	stats.GET("/soundtracks/most-played", h.statistics.GetMostPlayedSoundtracks)
 	stats.GET("/soundtracks/least-played", h.statistics.GetLeastPlayedSoundtracks)