fix(modpackchecker): Change check_interval validation from required to nullable

Disabled PRO fields don't submit values, causing validation error.
Now accepts null and defaults to 'daily' in update method.

Signed-off-by: Claude (Chronicler #63) <claude@firefrostgaming.com>

services/modpack-version-checker/blueprint-extension/admin/controller.php

@@ -67,7 +67,7 @@ class modpackcheckerSettingsFormRequest extends AdminFormRequest
         return [
             'curseforge_api_key' => 'nullable|string|max:500',
             'discord_webhook_url' => 'nullable|url|max:500',
-            'check_interval' => 'required|in:daily,12h,6h',
+            'check_interval' => 'nullable|in:daily,12h,6h',
         ];
     }