feat(mcp): MCP 密钥管理后台（CURD 重组 Base/App，密钥仅创建时明文展示）

database/migrations/20260816203002_system_mcp_key_node.php

@@ -0,0 +1,21 @@
+<?php
+
+use think\migration\Migrator;
+
+class SystemMcpKeyNode extends Migrator
+{
+    /**
+     * MCP密钥与节点关系表.
+     *
+     * 纯关系表：无任何时间戳字段。
+     */
+    public function change()
+    {
+        $table = $this->table('system_mcp_key_node')
+            ->setComment('MCP密钥与节点关系表')
+            ->addColumn('key_id', 'biginteger', ['limit' => 11, 'null' => true, 'signed' => false, 'comment' => '密钥ID'])
+            ->addColumn('node', 'char', ['limit' => 100, 'null' => true, 'default' => '', 'comment' => ''])
+            ->addIndex(['key_id'], ['name' => 'key_id'])
+            ->create();
+    }
+}