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

database/seeders/AdminRbacAndUserSeeder.php

@@ -7,6 +7,7 @@ use Illuminate\Database\Seeder;
 use App\Support\AdminAgentPermissionMenuActionSync;
 use App\Support\AdminDrawPermissionMenuActionSync;
 use App\Support\PlatformSystemRoles;
+use App\Support\SuperAdminAccount;
 
 /**
  * 后台 RBAC：平台固定角色 super_admin / agent。
@@ -22,8 +23,6 @@ final class AdminRbacAndUserSeeder extends Seeder
 
         PlatformSystemRoles::ensureAll();
 
-        $super = PlatformSystemRoles::ensureSuperAdminRole();
-
         $username = 'admin';
         AdminUser::query()->updateOrCreate(
             ['username' => $username],
@@ -37,13 +36,6 @@ final class AdminRbacAndUserSeeder extends Seeder
 
         /** @var AdminUser $admin */
         $admin = AdminUser::query()->where('username', $username)->firstOrFail();
-        $siteId = AdminUser::defaultAdminSiteId();
-        $superId = (int) $super->getKey();
-        $admin->roles()->sync([
-            $superId => [
-                'site_id' => $siteId,
-                'granted_at' => now(),
-            ],
-        ]);
+        SuperAdminAccount::assign($admin);
     }
 }