fixes and check for valid cron expressions

app/Models/InstanceSettings.php

@@ -18,6 +18,9 @@ class InstanceSettings extends Model implements SendsEmail
         'resale_license' => 'encrypted',
         'smtp_password' => 'encrypted',
         'allowed_ip_ranges' => 'array',
+        'is_auto_update_enabled' => 'boolean',
+        'auto_update_frequency' => 'string',
+        'update_check_frequency' => 'string',
     ];
 
     public function fqdn(): Attribute