fix(import): add validation for handle field to prevent Router.invalidParameterType error

fixes #119

modules/Admin/Controllers/PodcastImportController.php

@@ -65,6 +65,7 @@ class PodcastImportController extends BaseController
         helper(['media', 'misc']);
 
         $rules = [
+            'handle' => 'required|regex_match[/^[a-zA-Z0-9\_]{1,32}$/]',
             'imported_feed_url' => 'required|validate_url',
             'season_number' => 'is_natural_no_zero|permit_empty',
             'max_episodes' => 'is_natural_no_zero|permit_empty',