fix: allow registration_banner_url and registration_logo_url on event update

Missing from UpdateEventRequest rules, so the fields were stripped from
validated() and the uploaded URLs never persisted — the preview showed
briefly in the upload component but disappeared on reload because the
event record still had null.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/Http/Requests/Api/V1/UpdateEventRequest.php

@@ -29,6 +29,8 @@ final class UpdateEventRequest extends FormRequest
             'event_type_label' => ['nullable', 'string', 'max:50'],
             'sub_event_label' => ['nullable', 'string', 'max:50'],
             'registration_welcome_text' => ['nullable', 'string', 'max:1000'],
+            'registration_banner_url' => ['nullable', 'string', 'max:2048'],
+            'registration_logo_url' => ['nullable', 'string', 'max:2048'],
             'registration_show_section_preferences' => ['nullable', 'boolean'],
             'registration_show_availability' => ['nullable', 'boolean'],
         ];