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

tests/Feature/AdminAuthLoginTest.php

@@ -21,33 +21,7 @@ test('admin auth me returns current admin profile', function () {
         'email' => null,
         'password' => 'secret-strong',
         'status' => 0,
-    ]);
-
-    $roleId = DB::table('admin_roles')->insertGetId([
-        'code' => 'super_admin',
-        'slug' => 'super_admin',
-        'name' => '超级管理员',
-        'description' => null,
-        'status' => 1,
-        'is_system' => true,
-        'sort_order' => 0,
-        'created_at' => now(),
-        'updated_at' => now(),
-    ]);
-    $siteId = DB::table('admin_sites')->insertGetId([
-        'code' => 'default',
-        'name' => '默认站点',
-        'is_default' => true,
-        'status' => 1,
-        'created_at' => now(),
-        'updated_at' => now(),
-    ]);
-
-    DB::table('admin_user_site_roles')->insert([
-        'admin_user_id' => $admin->id,
-        'site_id' => $siteId,
-        'role_id' => $roleId,
-        'granted_at' => now(),
+        'is_super_admin' => true,
     ]);
 
     $token = $admin->createToken('admin-api', ['*'], now()->addDay())->plainTextToken;