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_integration_test.go

@@ -57,7 +57,11 @@ func TestSyncWithPostgres(t *testing.T) {
 
 	// Play a song, then rename one of its files and, in a later sync, the soundtrack folder.
 	// (Renaming both at once changes the content hash, so the folder would count as new.)
-	require.NoError(t, q.AddSongPlayed(ctx, repository.AddSongPlayedParams{SoundtrackID: bird.ID, SongName: "2. Yesterday"}))
+	for _, song := range songs {
+		if song.SongName == "2. Yesterday" {
+			require.NoError(t, q.AddSongPlayedByUUID(ctx, song.Uuid))
+		}
+	}
 	birdDir := filepath.Join(musicDir, "A Bird Story")
 	require.NoError(t, os.Rename(filepath.Join(birdDir, "2. Yesterday.mp3"), filepath.Join(birdDir, "02 Yesterday.mp3")))