test: add e2e test matching reporter's exact library layout (#5456)

Adds a deeply nested test (Genre/Artist/Album/Disc) with 12 discs
using the reporter's actual folder names to verify artwork resolution
works for non-top-level album folders too.

core/artwork/e2e/disc_test.go

@@ -257,6 +257,61 @@ var _ = Describe("Disc artwork resolution", func() {
 	})
 
 	// Reproduces https://github.com/navidrome/navidrome/issues/5456
+	// Deeply nested layout matching the reporter's actual structure.
+	When("a deeply nested multi-disc album has cover.jpg and per-disc folder.jpg", func() {
+		// Genre/Artist/Album/                 ← album root with cover.jpg
+		// ├── cover.jpg                       ← album-level cover
+		// ├── Disc 01 (Subtitle)/
+		// │   ├── 01 - Track.mp3
+		// │   └── folder.jpg                  ← disc 1 art
+		// ├── Disc 02 (Subtitle)/
+		// │   ├── 01 - Track.mp3
+		// │   └── folder.jpg
+		// └── ... (12 discs)
+		It("uses album-root cover.jpg for album art and per-disc folder.jpg for each disc", func() {
+			conf.Server.DiscArtPriority = defaultDiscPriority
+			conf.Server.CoverArtPriority = defaultCoverPriority
+			discNames := []string{
+				"Disc 01 (Birth of the Dead - The Studio Sides)",
+				"Disc 02 (Birth of the Dead - The Live Sides)",
+				"Disc 03 (The Grateful Dead)",
+				"Disc 04 (Anthem of the Sun)",
+				"Disc 05 (Aoxomoxoa)",
+				"Disc 06 (Live; Dead)",
+				"Disc 07 (Workingman's Dead)",
+				"Disc 08 (American Beauty)",
+				"Disc 09 (Grateful Dead)",
+				"Disc 10 (Europe '72)",
+				"Disc 11 (Europe '72)",
+				"Disc 12 (History of the Grateful Dead, Volume One (Bear's Choice))",
+			}
+			layout := fstest.MapFS{
+				"Pop; Rock/Grateful Dead/(2001) The Golden Road/cover.jpg": imageFile("album-root-cover"),
+			}
+			for i, name := range discNames {
+				discNum := i + 1
+				prefix := fmt.Sprintf("Pop; Rock/Grateful Dead/(2001) The Golden Road/%s/", name)
+				layout[prefix+"01 - Track.mp3"] = trackFile(1, fmt.Sprintf("T%d", discNum), map[string]any{"disc": fmt.Sprintf("%d", discNum)})
+				layout[prefix+"folder.jpg"] = imageFile(fmt.Sprintf("disc-%02d-folder", discNum))
+			}
+			setLayout(layout)
+			scan()
+
+			al := firstAlbum()
+
+			Expect(readArtwork(al.CoverArtID())).To(Equal(imageBytes("album-root-cover")))
+
+			for i := range discNames {
+				discNum := i + 1
+				discID := model.NewArtworkID(model.KindDiscArtwork, model.DiscArtworkID(al.ID, discNum), &al.UpdatedAt)
+				Expect(readArtwork(discID)).To(Equal(imageBytes(fmt.Sprintf("disc-%02d-folder", discNum))),
+					"disc %d should use its own folder.jpg", discNum)
+			}
+		})
+	})
+
+	// https://github.com/navidrome/navidrome/issues/5456
+	// Top-level album variant — album folder at library root (Path=".").
 	When("a top-level multi-disc album has cover.jpg and per-disc folder.jpg", func() {
 		// Album/                       (top-level, Path=".")
 		// ├── cover.jpg                ← album-level cover