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/asynq_runner.go

@@ -54,6 +54,7 @@ func (r *Runner) runAsynq(ctx context.Context) error {
 	mux.HandleFunc(asynqqueue.TypeAudioGenerate, r.handleAudioTask)
 	mux.HandleFunc(asynqqueue.TypeScrapeBook, r.handleScrapeTask)
 	mux.HandleFunc(asynqqueue.TypeScrapeCatalogue, r.handleScrapeTask)
+	mux.HandleFunc(asynqqueue.TypeImportBook, r.handleImportTask)
 
 	// Register Asynq queue metrics with the default Prometheus registry so
 	// the /metrics endpoint (metrics.go) can expose them.
@@ -191,6 +192,24 @@ func (r *Runner) handleAudioTask(ctx context.Context, t *asynq.Task) error {
 	return nil
 }
 
+// handleImportTask is the Asynq handler for TypeImportBook (PDF/EPUB import).
+func (r *Runner) handleImportTask(ctx context.Context, t *asynq.Task) error {
+	var p asynqqueue.ImportPayload
+	if err := json.Unmarshal(t.Payload(), &p); err != nil {
+		return fmt.Errorf("unmarshal import payload: %w", err)
+	}
+	task := domain.ImportTask{
+		ID:        p.PBTaskID,
+		Slug:      p.Slug,
+		Title:     p.Title,
+		FileType:  p.FileType,
+	}
+	r.tasksRunning.Add(1)
+	defer r.tasksRunning.Add(-1)
+	r.runImportTask(ctx, task, p.ObjectKey)
+	return nil
+}
+
 // pollTranslationTasks claims all available translation tasks from PocketBase
 // and dispatches them to goroutines. Translation tasks don't go through Redis/Asynq
 // because they're stored in PocketBase, so we need this separate poll loop.