feat: add PDF/EPUB import functionality

- Add ImportTask/ImportResult types to domain.go
- Add TypeImportBook to asynqqueue for task routing
- Add CreateImportTask to producer and storage layers
- Add ClaimNextImportTask/FinishImportTask to Consumer interfaces
- Add import task handling to runner (polling + Asynq handler)
- Add BookImporter interface to bookstore for PDF/EPUB parsing
- Add backend API endpoints: POST/GET /api/admin/import
- Add SvelteKit UI at /admin/import with task list
- Add nav link in admin layout

Note: PDF/EPUB parsing is a placeholder - needs external library integration.

backend/internal/runner/runner_test.go

@@ -54,6 +54,10 @@ func (s *stubConsumer) ClaimNextTranslationTask(_ context.Context, _ string) (do
 	return domain.TranslationTask{}, false, nil
 }
 
+func (s *stubConsumer) ClaimNextImportTask(_ context.Context, _ string) (domain.ImportTask, bool, error) {
+	return domain.ImportTask{}, false, nil
+}
+
 func (s *stubConsumer) FinishScrapeTask(_ context.Context, id string, _ domain.ScrapeResult) error {
 	s.finished = append(s.finished, id)
 	return nil
@@ -69,6 +73,11 @@ func (s *stubConsumer) FinishTranslationTask(_ context.Context, id string, _ dom
 	return nil
 }
 
+func (s *stubConsumer) FinishImportTask(_ context.Context, id string, _ domain.ImportResult) error {
+	s.finished = append(s.finished, id)
+	return nil
+}
+
 func (s *stubConsumer) FailTask(_ context.Context, id, _ string) error {
 	s.failCalled = append(s.failCalled, id)
 	return nil