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/backend/sync_test.go

@@ -229,7 +229,7 @@ func TestSyncDetectsRenamedSoundtrack(t *testing.T) {
 	f.write("Game A/01.mp3", "a1")
 	f.run(false)
 	a := f.soundtrack("Game A")
-	require.NoError(t, f.store.AddSongPlayed(context.Background(), repository.AddSongPlayedParams{SoundtrackID: a.ID, SongName: "01"}))
+	require.NoError(t, f.store.AddSongPlayedByUUID(context.Background(), f.store.SongsOf(a.ID)[0].Uuid))
 
 	require.NoError(t, os.Rename(filepath.Join(f.dir, "Game A"), filepath.Join(f.dir, "Game A Remastered")))
 	result := f.run(false)