fix(scanner): add nil guards to cursor wrapping (#5139)

* fix(persistence): add nil guards to cursor wrapping in folder and mediafile repos

Prevent SIGSEGV panic when queryWithStableResults yields a zero-value
struct on the rows.Err() path (e.g., "database is locked" during
concurrent scanning). Extract cursor wrapping into wrapFolderCursor and
wrapMediaFileCursor with nil checks matching the existing pattern in
album_repository.go.

Fixes #5138

* fix(persistence): wrap original cursor error in nil guard messages

Use %w to preserve the underlying error (e.g., "database is locked")
so callers can use errors.Is/As for root cause analysis. Tests now
verify the original error is accessible via errors.Is.

* fix(persistence): add nil guards and error wrapping in album, folder, and mediafile cursor functions

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

---------

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

persistence/folder_repository_test.go

@@ -2,6 +2,7 @@ package persistence
 
 import (
 	"context"
+	"errors"
 	"fmt"
 
 	"github.com/navidrome/navidrome/log"
@@ -210,4 +211,44 @@ var _ = Describe("FolderRepository", func() {
 			})
 		})
 	})
+
+	Describe("wrapFolderCursor", func() {
+		It("does not panic when the cursor yields a dbFolder with nil Folder", func() {
+			// Simulate what queryWithStableResults does on the rows.Err() path:
+			// it yields a zero-value dbFolder (where Folder is nil) with an error.
+			dbErr := fmt.Errorf("database is locked")
+			cursor := func(yield func(dbFolder, error) bool) {
+				var empty dbFolder // Folder pointer is nil
+				yield(empty, dbErr)
+			}
+
+			// wrapFolderCursor should handle the nil Folder without panicking
+			wrappedCursor := wrapFolderCursor(cursor)
+			var gotErr error
+			Expect(func() {
+				for _, err := range wrappedCursor {
+					gotErr = err
+				}
+			}).ToNot(Panic())
+			Expect(gotErr).To(HaveOccurred())
+			Expect(gotErr.Error()).To(ContainSubstring("unexpected nil folder"))
+			Expect(errors.Is(gotErr, dbErr)).To(BeTrue(), "should wrap the original cursor error")
+		})
+
+		It("yields folders from a valid cursor", func() {
+			folder := &model.Folder{ID: "f1", Name: "Test"}
+			cursor := func(yield func(dbFolder, error) bool) {
+				yield(dbFolder{Folder: folder}, nil)
+			}
+
+			wrappedCursor := wrapFolderCursor(cursor)
+			var folders []model.Folder
+			for f, err := range wrappedCursor {
+				Expect(err).ToNot(HaveOccurred())
+				folders = append(folders, f)
+			}
+			Expect(folders).To(HaveLen(1))
+			Expect(folders[0].ID).To(Equal("f1"))
+		})
+	})
 })