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/library_test.go

@@ -0,0 +1,82 @@
+package backend
+
+import (
+	"context"
+	"os"
+	"testing"
+	"time"
+
+	"music-server/internal/db/repository"
+	"music-server/internal/testutil/fakedb"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestLibrarySoundtracks(t *testing.T) {
+	store := fakedb.New()
+	b := store.AddSoundtrack(repository.Soundtrack{SoundtrackName: "B Game", TimesPlayed: 3})
+	a := store.AddSoundtrack(repository.Soundtrack{SoundtrackName: "A Game"})
+	deleted := time.Now()
+	store.AddSoundtrack(repository.Soundtrack{SoundtrackName: "Gone", Deleted: &deleted})
+
+	soundtracks, err := NewLibraryService(store).Soundtracks(context.Background())
+	require.NoError(t, err)
+	require.Len(t, soundtracks, 2)
+	assert.Equal(t, Soundtrack{Uuid: a.Uuid, Name: "A Game"}, soundtracks[0])
+	assert.Equal(t, b.Uuid, soundtracks[1].Uuid)
+	assert.Equal(t, int32(3), soundtracks[1].TimesPlayed)
+}
+
+func TestLibrarySoundtrackWithSongs(t *testing.T) {
+	store := fakedb.New()
+	st := addSoundtrackWithSongs(t, store, t.TempDir(), "Game A", 0, "Song B", "Song A")
+	l := NewLibraryService(store)
+
+	result, err := l.Soundtrack(context.Background(), st.Uuid)
+	require.NoError(t, err)
+	assert.Equal(t, "Game A", result.Name)
+	require.Len(t, result.Songs, 2)
+	assert.Equal(t, "Song A", result.Songs[0].Name)
+	assert.Equal(t, st.Uuid, result.Songs[0].SoundtrackUuid)
+	assert.Equal(t, "Game A", result.Songs[0].SoundtrackName)
+
+	_, err = l.Soundtrack(context.Background(), uuid.New())
+	assert.ErrorIs(t, err, ErrNotFound)
+}
+
+func TestLibrarySong(t *testing.T) {
+	store := fakedb.New()
+	st := addSoundtrackWithSongs(t, store, t.TempDir(), "Game A", 0, "Song")
+	song := store.SongsOf(st.ID)[0]
+	l := NewLibraryService(store)
+	ctx := context.Background()
+
+	result, err := l.Song(ctx, song.Uuid)
+	require.NoError(t, err)
+	assert.Equal(t, Song{Uuid: song.Uuid, Name: "Song", FileName: song.FileName, SoundtrackUuid: st.Uuid, SoundtrackName: "Game A"}, result)
+
+	path, err := l.SongPath(ctx, song.Uuid)
+	require.NoError(t, err)
+	assert.Equal(t, song.Path, path)
+
+	require.NoError(t, os.Remove(song.Path))
+	_, err = l.SongPath(ctx, song.Uuid)
+	assert.ErrorIs(t, err, ErrNotFound, "missing files are not served")
+
+	_, err = l.Song(ctx, uuid.New())
+	assert.ErrorIs(t, err, ErrNotFound)
+}
+
+func TestLibraryHidesSongsOfDeletedSoundtracks(t *testing.T) {
+	store := fakedb.New()
+	st := addSoundtrackWithSongs(t, store, t.TempDir(), "Game A", 0, "Song")
+	song := store.SongsOf(st.ID)[0]
+	require.NoError(t, store.MarkSoundtracksDeletedExcept(context.Background(), nil))
+
+	_, err := NewLibraryService(store).Song(context.Background(), song.Uuid)
+	assert.ErrorIs(t, err, ErrNotFound)
+	_, err = NewLibraryService(store).Soundtrack(context.Background(), st.Uuid)
+	assert.ErrorIs(t, err, ErrNotFound)
+}