Add GET /api/v1/statistics/soundtracks with every soundtrack and its songs

The other soundtrack statistics are capped at 100, so a client that shows every soundtrack (the MusicPlayer statistics tab) had no way to get them all. This one has no limit and is sorted by name.

internal/backend/library_integration_test.go

@@ -70,6 +70,22 @@ func TestLibraryAndStatisticsWithPostgres(t *testing.T) {
 	assert.Equal(t, song.Uuid, songs[0].SongUuid)
 	assert.Equal(t, bird.Uuid, songs[0].SoundtrackUuid)
 
+	all, err := stats.AllSoundtracks(ctx)
+	require.NoError(t, err)
+	require.NotEmpty(t, all)
+	for i := 1; i < len(all); i++ {
+		assert.LessOrEqual(t, all[i-1].SoundtrackName, all[i].SoundtrackName, "sorted by name")
+	}
+	for _, st := range all {
+		if st.SoundtrackUuid == bird.Uuid {
+			assert.Equal(t, int32(1), st.SoundtrackPlayed)
+			assert.Len(t, st.Songs, 6)
+		}
+		if st.SoundtrackName == "Empty Soundtrack" {
+			assert.Empty(t, st.Songs, "a soundtrack without songs has an empty song list")
+		}
+	}
+
 	never, err := stats.NeverPlayedSoundtracks(ctx)
 	require.NoError(t, err)
 	for _, st := range never {