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/Support/SitePlatformRole.php

@@ -0,0 +1,54 @@
+<?php
+
+namespace App\Support;
+
+use App\Models\AdminRole;
+use App\Models\AdminUser;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Validation\ValidationException;
+
+/** 接入站点后台账号统一使用平台系统角色 slug=site_admin。 */
+final class SitePlatformRole
+{
+    public const SLUG = 'site_admin';
+
+    public static function resolve(): AdminRole
+    {
+        return SiteAdminDefaultRolePermissions::ensurePlatformSiteAdminRole();
+    }
+
+    public static function id(): int
+    {
+        return (int) self::resolve()->id;
+    }
+
+    public static function idOrFail(): int
+    {
+        $id = (int) (AdminRole::query()
+            ->where('scope_type', AdminRole::SCOPE_SYSTEM)
+            ->where('slug', self::SLUG)
+            ->where('status', 1)
+            ->value('id') ?? 0);
+
+        if ($id <= 0) {
+            throw ValidationException::withMessages([
+                'role' => ['platform_site_admin_role_missing: run php artisan lottery:admin-auth-sync'],
+            ]);
+        }
+
+        return $id;
+    }
+
+    public static function userHasSiteAdminRole(AdminUser $user): bool
+    {
+        if ($user->isSuperAdmin() || $user->hasPrimaryAgentBinding()) {
+            return false;
+        }
+
+        return DB::table('admin_user_site_roles as usr')
+            ->join('admin_roles as r', 'r.id', '=', 'usr.role_id')
+            ->where('usr.admin_user_id', $user->id)
+            ->where('r.slug', self::SLUG)
+            ->exists();
+    }
+}