fix(artwork): treat playlist cover URL 404 as definitive miss

The playlist ExternalImageURL step used sources.go's fromURL, which maps any
non-200 to a generic error, so a stale URL returning 404/410 was classified
transient: infinite backoff plus it counted toward the circuit breaker,
blocking valid external work. Add a local fetch in resolve.go that maps
404/410 to model.ErrNotFound (definitive) while keeping other non-200s
transient. sources.go is left untouched.

core/artwork/resolve.go

@@ -10,12 +10,15 @@ import (
 	"image/png"
 	"io"
 	"io/fs"
+	"net/http"
 	"net/url"
 	"os"
 	"strings"
+	"time"
 
 	"github.com/Masterminds/squirrel"
 	"github.com/navidrome/navidrome/conf"
+	"github.com/navidrome/navidrome/consts"
 	"github.com/navidrome/navidrome/core/external"
 	"github.com/navidrome/navidrome/core/ffmpeg"
 	"github.com/navidrome/navidrome/model"
@@ -283,7 +286,7 @@ func fromPlaylistExternalSource(ctx context.Context, pl model.Playlist) sourceFu
 			if !conf.Server.EnableM3UExternalAlbumArt {
 				return nil, "", nil
 			}
-			return fromURL(ctx, parsed)
+			return fetchPlaylistImageURL(ctx, parsed)
 		}
 		// A missing/unreadable local file is a definitive miss, not a transient
 		// failure to retry: swallow the open error and fall through to the grid.
@@ -292,6 +295,27 @@ func fromPlaylistExternalSource(ctx context.Context, pl model.Playlist) sourceFu
 	}
 }
 
+// Like sources.go's fromURL but maps 404/410 to ErrNotFound (definitive), so a stale M3U
+// cover URL falls through to the grid instead of retrying forever and tripping the breaker.
+func fetchPlaylistImageURL(ctx context.Context, imageURL *url.URL) (io.ReadCloser, string, error) {
+	hc := http.Client{Timeout: 5 * time.Second}
+	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, imageURL.String(), nil)
+	req.Header.Set("User-Agent", consts.HTTPUserAgent)
+	resp, err := hc.Do(req) //nolint:gosec
+	if err != nil {
+		return nil, "", err
+	}
+	if resp.StatusCode == http.StatusNotFound || resp.StatusCode == http.StatusGone {
+		resp.Body.Close()
+		return nil, "", model.ErrNotFound
+	}
+	if resp.StatusCode != http.StatusOK {
+		resp.Body.Close()
+		return nil, "", fmt.Errorf("error retrieving artwork from %s: %s", imageURL, resp.Status)
+	}
+	return resp.Body, imageURL.String(), nil
+}
+
 func resolveEmbedded(ctx context.Context, lib libraryView, ffm ffmpeg.FFmpeg, embedRel string) (resolution, bool) {
 	if embedRel == "" {
 		return resolution{}, false

core/artwork/resolve_test.go

@@ -5,6 +5,8 @@ import (
 	"errors"
 	"image"
 	"io"
+	"net/http"
+	"net/http/httptest"
 	"net/url"
 	"os"
 	"path/filepath"
@@ -452,6 +454,45 @@ var _ = Describe("resolveItem", func() {
 			Expect(res.extError).To(BeFalse())
 		})
 
+		It("treats an ExternalImageURL 404 as a definitive miss and falls through to the grid", func() {
+			conf.Server.EnableM3UExternalAlbumArt = true
+			srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+				w.WriteHeader(http.StatusNotFound)
+			}))
+			defer srv.Close()
+
+			plRepo := tests.CreateMockPlaylistRepo()
+			plRepo.SetData(model.Playlists{{ID: "pl404", Name: "Playlist", ExternalImageURL: srv.URL}})
+			plRepo.TracksRepo = &tests.MockPlaylistTrackRepo{AlbumIDs: []string{"t1"}}
+			ds.MockedPlaylist = plRepo
+
+			res, err := resolveItem(ctx, ds, prov, ffm, model.ArtworkQueueItem{ItemKind: "pl", ItemID: "pl404"}, nil)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(res.reader).ToNot(BeNil())
+			defer res.reader.Close()
+			Expect(res.source).To(Equal("generated"))
+			Expect(res.extError).To(BeFalse())
+		})
+
+		It("treats an ExternalImageURL 500 as a transient failure and sets extError", func() {
+			conf.Server.EnableM3UExternalAlbumArt = true
+			folderRepo.result = nil // no grid tiles, so the external failure is what surfaces
+			srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+				w.WriteHeader(http.StatusInternalServerError)
+			}))
+			defer srv.Close()
+
+			plRepo := tests.CreateMockPlaylistRepo()
+			plRepo.SetData(model.Playlists{{ID: "pl500", Name: "Playlist", ExternalImageURL: srv.URL}})
+			plRepo.TracksRepo = &tests.MockPlaylistTrackRepo{AlbumIDs: []string{"t1"}}
+			ds.MockedPlaylist = plRepo
+
+			res, err := resolveItem(ctx, ds, prov, ffm, model.ArtworkQueueItem{ItemKind: "pl", ItemID: "pl500"}, nil)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(res.reader).To(BeNil())
+			Expect(res.extError).To(BeTrue())
+		})
+
 		It("yields an empty resolution when no album has art", func() {
 			ds.MockedAlbum.(*tests.MockAlbumRepo).SetData(model.Albums{
 				{ID: "empty1", Name: "Empty"},