feat: refactor super admin to use is_super_admin flag and enhance site deletion logic

- Changed super admin detection from role-based to `is_super_admin` flag in AdminUser model
- Added `requireDefaultAdminSiteId()` method to throw validation error when no integration site exists
- Enhanced site deletion to migrate platform role bindings to fallback site and auto-delete site-specific admin accounts
- Made agent line code optional with auto-generation fallback using `{site_code}-agent-{counter}` format

app/Http/Controllers/Api/V1/Admin/User/AdminUserPermissionSyncController.php

@@ -25,7 +25,7 @@ final class AdminUserPermissionSyncController extends Controller
             (array) ($input['permissions'] ?? $input['permission_slugs'] ?? []),
             static fn ($v) => is_string($v) && $v !== '',
         )));
-        $siteId = AdminUser::defaultAdminSiteId();
+        $siteId = AdminUser::requireDefaultAdminSiteId();
 
         $codes = [];
         foreach ($slugs as $slug) {