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/db/repository/querier.go

@@ -15,7 +15,9 @@ type Querier interface {
 	AddHashToSong(ctx context.Context, arg AddHashToSongParams) error
 	AddSong(ctx context.Context, arg AddSongParams) error
 	AddSongPlayed(ctx context.Context, arg AddSongPlayedParams) error
+	AddSongPlayedByUUID(ctx context.Context, argUuid uuid.UUID) error
 	AddSoundtrackPlayed(ctx context.Context, id int32) error
+	AddSoundtrackPlayedByUUID(ctx context.Context, argUuid uuid.UUID) error
 	CheckSong(ctx context.Context, arg CheckSongParams) (int64, error)
 	CheckSongWithHash(ctx context.Context, hash string) (int64, error)
 	ClearSongs(ctx context.Context) error
@@ -27,6 +29,7 @@ type Querier interface {
 	FetchAllSongs(ctx context.Context) ([]Song, error)
 	FindAllSoundtracks(ctx context.Context) ([]Soundtrack, error)
 	FindSongsFromSoundtrack(ctx context.Context, soundtrackID int32) ([]Song, error)
+	GetActiveSongByUUID(ctx context.Context, argUuid uuid.UUID) (GetActiveSongByUUIDRow, error)
 	GetAllSoundtracksIncludingDeleted(ctx context.Context) ([]Soundtrack, error)
 	GetIdBySoundtrackName(ctx context.Context, soundtrackName string) (int32, error)
 	// Last played soundtracks (most recently played)