fix: add DeduplicateChapters stub to test mocks to satisfy BookWriter interface

backend/internal/runner/runner_test.go

@@ -94,6 +94,10 @@ func (s *stubBookWriter) ChapterExists(_ context.Context, _ string, _ domain.Cha
 	return false
 }
 
+func (s *stubBookWriter) DeduplicateChapters(_ context.Context, _ string) (int, error) {
+	return 0, nil
+}
+
 // stubBookReader satisfies bookstore.BookReader — returns a single chapter.
 type stubBookReader struct {
 	text    string