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/music_test.go

@@ -9,6 +9,7 @@ import (
 	"music-server/internal/db/repository"
 	"music-server/internal/testutil/fakedb"
 
+	"github.com/google/uuid"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -84,6 +85,9 @@ func TestMusicServiceQueue(t *testing.T) {
 
 	info := m.SongInfo(ctx)
 	assert.Equal(t, "Game A", info.Soundtrack)
+	soundtrack, _ := store.Soundtrack("Game A")
+	assert.Equal(t, soundtrack.Uuid, info.SoundtrackUuid)
+	assert.Contains(t, []string{first, second}, songPathByUUID(store, info.SongUuid))
 	assert.Equal(t, 1, info.SongNo)
 	assert.True(t, info.CurrentlyPlaying)
 
@@ -222,3 +226,32 @@ func TestSoundCheckSong(t *testing.T) {
 	_, err = NewMusicService(fakedb.New(), filepath.Join(dir, "missing")).SoundCheckSong()
 	assert.Error(t, err)
 }
+
+func songPathByUUID(store *fakedb.Store, id uuid.UUID) string {
+	for _, song := range store.Songs {
+		if song.Uuid == id {
+			return song.Path
+		}
+	}
+	return ""
+}
+
+func TestMarkPlayedUsesSongUUID(t *testing.T) {
+	// Two songs with the same name in one soundtrack: only the played one is counted.
+	store := fakedb.New()
+	st := addSoundtrackWithSongs(t, store, t.TempDir(), "Game A", 0, "Song")
+	fileName := "Song (copy).mp3"
+	writeFile(t, st.Path+fileName, "copy")
+	store.AddTestSong(repository.Song{SoundtrackID: st.ID, SongName: "Song", Path: st.Path + fileName, FileName: &fileName})
+	m := NewMusicService(store, t.TempDir())
+	ctx := context.Background()
+
+	_, err := m.RandomSong(ctx)
+	require.NoError(t, err)
+	m.AddLatestToQueue()
+	require.NoError(t, m.AddLatestPlayed(ctx))
+
+	songs := store.SongsOf(st.ID)
+	require.Len(t, songs, 2)
+	assert.ElementsMatch(t, []int32{0, 1}, []int32{songs[0].TimesPlayed, songs[1].TimesPlayed})
+}