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/Requests/Admin/AdminAgentLineStoreRequest.php

@@ -40,7 +40,7 @@ final class AdminAgentLineStoreRequest extends ApiFormRequest
     {
         return [
             'site_code' => ['required', 'string', 'max:64', 'regex:/^[a-z0-9][a-z0-9_-]*$/', Rule::exists('admin_sites', 'code')],
-            'code' => ['required', 'string', 'max:64', 'regex:/^[a-z0-9][a-z0-9_-]*$/', Rule::unique('agent_nodes', 'code')],
+            'code' => ['sometimes', 'nullable', 'string', 'max:64', 'regex:/^[a-z0-9][a-z0-9_-]*$/', Rule::unique('agent_nodes', 'code')],
             'name' => ['required', 'string', 'max:128'],
             'username' => ['required', 'string', 'max:64', Rule::unique('admin_users', 'username')],
             'password' => ['required', 'string', 'min:8', 'max:128'],