Move engine package under subsonic, as it should only be used by the Subsonic API.master

The idea is to move reusable code from `engine` to `core`, in future refactorings

server/subsonic/engine/mock_transcoding_repo_test.go

@@ -0,0 +1,22 @@
+package engine
+
+import "github.com/deluan/navidrome/model"
+
+type mockTranscodingRepository struct {
+	model.TranscodingRepository
+}
+
+func (m *mockTranscodingRepository) Get(id string) (*model.Transcoding, error) {
+	return &model.Transcoding{ID: id, TargetFormat: "mp3", DefaultBitRate: 160}, nil
+}
+
+func (m *mockTranscodingRepository) FindByFormat(format string) (*model.Transcoding, error) {
+	switch format {
+	case "mp3":
+		return &model.Transcoding{ID: "mp31", TargetFormat: "mp3", DefaultBitRate: 160}, nil
+	case "oga":
+		return &model.Transcoding{ID: "oga1", TargetFormat: "oga", DefaultBitRate: 128}, nil
+	default:
+		return nil, model.ErrNotFound
+	}
+}