feat: 后端全量更新 - 含所有本次需求

- Contract.php: 返回合约账户余额(balance_contract)
- My.php: 地址管理增加BTC/ETH
- AppContract.php: 一键平仓(closeall)
- AppProxy.php: 代理专属注册链接 + 分级权限(L1/L2)
- site.php: 手续费减半(0.018→0.009)
- agent_permission_setup.sql: 代理权限SQL
- crypto_news_crawler.py: 新闻自动采集脚本

thinkphp/library/think/db/Expression.php

@@ -0,0 +1,48 @@
+<?php
+// +----------------------------------------------------------------------
+// | ThinkPHP [ WE CAN DO IT JUST THINK ]
+// +----------------------------------------------------------------------
+// | Copyright (c) 2006~2018 http://thinkphp.cn All rights reserved.
+// +----------------------------------------------------------------------
+// | Licensed ( http://www.apache.org/licenses/LICENSE-2.0 )
+// +----------------------------------------------------------------------
+// | Author: liu21st <liu21st@gmail.com>
+// +----------------------------------------------------------------------
+
+namespace think\db;
+
+class Expression
+{
+    /**
+     * 查询表达式
+     *
+     * @var string
+     */
+    protected $value;
+
+    /**
+     * 创建一个查询表达式
+     *
+     * @param  string  $value
+     * @return void
+     */
+    public function __construct($value)
+    {
+        $this->value = $value;
+    }
+
+    /**
+     * 获取表达式
+     *
+     * @return string
+     */
+    public function getValue()
+    {
+        return $this->value;
+    }
+
+    public function __toString()
+    {
+        return (string) $this->value;
+    }
+}