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/taskqueue/taskqueue_test.go

@@ -26,6 +26,9 @@ func (s *stubStore) CreateAudioTask(_ context.Context, _ string, _ int, _ string
 func (s *stubStore) CreateTranslationTask(_ context.Context, _ string, _ int, _ string) (string, error) {
 	return "translation-1", nil
 }
+func (s *stubStore) CreateImportTask(_ context.Context, _, _, _, _ string) (string, error) {
+	return "import-1", nil
+}
 func (s *stubStore) CancelTask(_ context.Context, _ string) error                    { return nil }
 func (s *stubStore) CancelAudioTasksBySlug(_ context.Context, _ string) (int, error) { return 0, nil }
 
@@ -38,6 +41,9 @@ func (s *stubStore) ClaimNextAudioTask(_ context.Context, _ string) (domain.Audi
 func (s *stubStore) ClaimNextTranslationTask(_ context.Context, _ string) (domain.TranslationTask, bool, error) {
 	return domain.TranslationTask{ID: "translation-1", Status: domain.TaskStatusRunning}, true, nil
 }
+func (s *stubStore) ClaimNextImportTask(_ context.Context, _ string) (domain.ImportTask, bool, error) {
+	return domain.ImportTask{ID: "import-1", Status: domain.TaskStatusRunning}, true, nil
+}
 func (s *stubStore) FinishScrapeTask(_ context.Context, _ string, _ domain.ScrapeResult) error {
 	return nil
 }
@@ -47,6 +53,9 @@ func (s *stubStore) FinishAudioTask(_ context.Context, _ string, _ domain.AudioR
 func (s *stubStore) FinishTranslationTask(_ context.Context, _ string, _ domain.TranslationResult) error {
 	return nil
 }
+func (s *stubStore) FinishImportTask(_ context.Context, _ string, _ domain.ImportResult) error {
+	return nil
+}
 func (s *stubStore) FailTask(_ context.Context, _, _ string) error { return nil }
 
 func (s *stubStore) HeartbeatTask(_ context.Context, _ string) error { return nil }
@@ -69,6 +78,10 @@ func (s *stubStore) ListTranslationTasks(_ context.Context) ([]domain.Translatio
 func (s *stubStore) GetTranslationTask(_ context.Context, _ string) (domain.TranslationTask, bool, error) {
 	return domain.TranslationTask{}, false, nil
 }
+func (s *stubStore) ListImportTasks(_ context.Context) ([]domain.ImportTask, error) { return nil, nil }
+func (s *stubStore) GetImportTask(_ context.Context, _ string) (domain.ImportTask, bool, error) {
+	return domain.ImportTask{}, false, nil
+}
 
 // Verify the stub satisfies all three interfaces at compile time.
 var _ taskqueue.Producer = (*stubStore)(nil)