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

public/assets/less/fastadmin/404_500_errors.less

@@ -0,0 +1,36 @@
+/*
+ * Page: 400 and 500 error pages
+ * ------------------------------
+ */
+.error-page {
+  width: 600px;
+  margin: 20px auto 0 auto;
+  @media (max-width: @screen-sm-max) {
+    width: 100%;
+  }
+  //For the error number e.g: 404
+  > .headline {
+    float: left;
+    font-size: 100px;
+    font-weight: 300;
+    @media (max-width: @screen-sm-max) {
+      float: none;
+      text-align: center;
+    }
+  }
+  //For the message
+  > .error-content {
+    margin-left: 190px;
+    @media (max-width: @screen-sm-max) {
+      margin-left: 0;
+    }
+    > h3 {
+      font-weight: 300;
+      font-size: 25px;
+      @media (max-width: @screen-sm-max) {
+        text-align: center;
+      }
+    }
+    display: block;
+  }
+}