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
- 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/go.mod

@@ -3,6 +3,7 @@ module github.com/libnovel/backend
 go 1.26.1
 
 require (
+	github.com/ledongthuc/pdf v0.0.0-20241014091450-14fc3c58b12d
 	github.com/minio/minio-go/v7 v7.0.98
 	golang.org/x/net v0.51.0
 )