fix(artwork): search parent folders for album cover art in multi-disc layouts (#5157)

* fix(artwork): search parent folders for album cover art in multi-disc layouts

When albums have tracks in subdirectories (e.g., CD1/, CD2/), Navidrome
only searched those subdirectories for cover images. This meant cover art
placed in the album's root folder (e.g., "Artist/Album/cover.jpg") was
not found. Now loadAlbumFoldersPaths also queries parent folders of the
album's media folders, so cover art in the album root is discovered.

* fix(artwork): simplify parent folder detection for album cover art lookup

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(album): propagate non-ErrNotFound errors from parent folder lookup

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

core/artwork/reader_artist_test.go

@@ -417,14 +417,28 @@ var _ = Describe("artistArtworkReader", func() {
 
 type fakeFolderRepo struct {
 	model.FolderRepository
-	result []model.Folder
-	err    error
+	result       []model.Folder
+	parentResult *model.Folder
+	getErr       error
+	getCallCount int
+	err          error
 }
 
 func (f *fakeFolderRepo) GetAll(...model.QueryOptions) ([]model.Folder, error) {
 	return f.result, f.err
 }
 
+func (f *fakeFolderRepo) Get(id string) (*model.Folder, error) {
+	f.getCallCount++
+	if f.getErr != nil {
+		return nil, f.getErr
+	}
+	if f.parentResult != nil {
+		return f.parentResult, nil
+	}
+	return nil, model.ErrNotFound
+}
+
 type fakeDataStore struct {
 	model.DataStore
 	folderRepo *fakeFolderRepo