feat(artwork): add external image URL source to playlist artwork reader

Add fromPlaylistExternalImage source function that resolves playlist
cover art from ExternalImageURL, supporting both HTTP(S) URLs (via
the existing fromURL helper) and local file paths (via os.Open).
Insert it in the Reader() fallback chain between sidecar and tiled cover.

core/artwork/artwork_internal_test.go

@@ -318,6 +318,44 @@ var _ = Describe("Artwork", func() {
 				Expect(path).To(BeEmpty())
 			})
 		})
+
+		Describe("fromPlaylistExternalImage", func() {
+			It("opens local path from ExternalImageURL", func() {
+				tmpDir := GinkgoT().TempDir()
+				imgPath := filepath.Join(tmpDir, "cover.jpg")
+				Expect(os.WriteFile(imgPath, []byte("external image data"), 0600)).To(Succeed())
+
+				reader := &playlistArtworkReader{
+					pl: model.Playlist{ExternalImageURL: imgPath},
+				}
+				r, path, err := reader.fromPlaylistExternalImage(ctx)()
+				Expect(err).ToNot(HaveOccurred())
+				Expect(r).ToNot(BeNil())
+				Expect(path).To(Equal(imgPath))
+				data, _ := io.ReadAll(r)
+				Expect(string(data)).To(Equal("external image data"))
+				r.Close()
+			})
+
+			It("returns nil when ExternalImageURL is empty", func() {
+				reader := &playlistArtworkReader{
+					pl: model.Playlist{ExternalImageURL: ""},
+				}
+				r, path, err := reader.fromPlaylistExternalImage(ctx)()
+				Expect(err).ToNot(HaveOccurred())
+				Expect(r).To(BeNil())
+				Expect(path).To(BeEmpty())
+			})
+
+			It("returns error when local file does not exist", func() {
+				reader := &playlistArtworkReader{
+					pl: model.Playlist{ExternalImageURL: "/non/existent/path/cover.jpg"},
+				}
+				r, _, err := reader.fromPlaylistExternalImage(ctx)()
+				Expect(err).To(HaveOccurred())
+				Expect(r).To(BeNil())
+			})
+		})
 	})
 
 	Describe("resizedArtworkReader", func() {

core/artwork/reader_playlist.go

@@ -8,6 +8,7 @@ import (
 	"image/draw"
 	"image/png"
 	"io"
+	"net/url"
 	"os"
 	"path/filepath"
 	"strings"
@@ -67,6 +68,7 @@ func (a *playlistArtworkReader) Reader(ctx context.Context) (io.ReadCloser, stri
 	return selectImageReader(ctx, a.artID,
 		a.fromPlaylistUploadedImage(),
 		a.fromPlaylistSidecar(),
+		a.fromPlaylistExternalImage(ctx),
 		a.fromGeneratedTiledCover(ctx),
 		fromAlbumPlaceholder(),
 	)
@@ -131,6 +133,26 @@ func (a *playlistArtworkReader) fromPlaylistSidecar() sourceFunc {
 	}
 }
 
+func (a *playlistArtworkReader) fromPlaylistExternalImage(ctx context.Context) sourceFunc {
+	return func() (io.ReadCloser, string, error) {
+		if a.pl.ExternalImageURL == "" {
+			return nil, "", nil
+		}
+		if strings.HasPrefix(a.pl.ExternalImageURL, "http://") || strings.HasPrefix(a.pl.ExternalImageURL, "https://") {
+			parsed, err := url.Parse(a.pl.ExternalImageURL)
+			if err != nil {
+				return nil, "", err
+			}
+			return fromURL(ctx, parsed)
+		}
+		f, err := os.Open(a.pl.ExternalImageURL)
+		if err != nil {
+			return nil, "", err
+		}
+		return f, a.pl.ExternalImageURL, nil
+	}
+}
+
 func (a *playlistArtworkReader) fromGeneratedTiledCover(ctx context.Context) sourceFunc {
 	return func() (io.ReadCloser, string, error) {
 		tiles, err := a.loadTiles(ctx)