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

backend/internal/orchestrator/orchestrator_test.go

@@ -89,6 +89,8 @@ func (s *stubStore) WriteChapterRefs(_ context.Context, _ string, _ []domain.Cha
 	return nil
 }
 
+func (s *stubStore) DeduplicateChapters(_ context.Context, _ string) (int, error) { return 0, nil }
+
 func (s *stubStore) ChapterExists(_ context.Context, slug string, ref domain.ChapterRef) bool {
 	s.mu.Lock()
 	defer s.mu.Unlock()