feat: 集成MCP统一外部接口管理系统

- 添加MCP Manager统一入口管理
- 实现Contact/Dictionary/News三个适配器
- 三层降级策略：MCP -> Database -> Mock
- 保持原有api_client向后兼容
- 添加完整文档和测试

backend/app/mcp/mcp_example.py

@@ -0,0 +1,87 @@
+"""
+MCP集成示例
+展示如何使用统一的MCP接口
+"""
+import asyncio
+from ..mcp.mcp_manager import mcp_manager
+from ..mcp.adapters import ContactAdapter, DictionaryAdapter, NewsAdapter
+
+
+async def setup_mcp():
+    """设置MCP系统"""
+    # 1. 配置MCP服务器（可选）
+    servers_config = {
+        "news": {
+            "type": "stdio",
+            "command": "npx",
+            "args": ["-y", "@modelcontextprotocol/server-news"],
+            "enabled": False  # 先禁用，等配置好后启用
+        },
+        "dictionary": {
+            "type": "stdio",
+            "command": "npx",
+            "args": ["-y", "@modelcontextprotocol/server-dictionary"],
+            "enabled": False
+        },
+        "email": {
+            "type": "stdio",
+            "command": "npx",
+            "args": ["-y", "@modelcontextprotocol/server-gmail"],
+            "enabled": False
+        }
+    }
+    mcp_manager.configure_servers(servers_config)
+    
+    # 2. 注册适配器
+    mcp_manager.register_adapter(ContactAdapter())
+    mcp_manager.register_adapter(DictionaryAdapter())
+    mcp_manager.register_adapter(NewsAdapter())
+    
+    # 3. 初始化
+    await mcp_manager.initialize()
+
+
+async def example_usage():
+    """使用示例"""
+    await setup_mcp()
+    
+    print("=" * 60)
+    print("可用适配器:", mcp_manager.get_available_adapters())
+    print("可用MCP工具:", mcp_manager.get_available_tools())
+    print("=" * 60)
+    
+    # 1. 查询词典
+    print("\n📖 查询单词 'ephemeral':")
+    result = await mcp_manager.execute(
+        "dictionary",
+        "query_word",
+        word="ephemeral",
+        user_id="default"
+    )
+    print(f"来源: {result.source}")
+    print(f"结果: {result.data}")
+    
+    # 2. 查询新闻
+    print("\n📰 查询新闻 'AI':")
+    result = await mcp_manager.execute(
+        "news",
+        "query_news",
+        query="AI",
+        user_id="default"
+    )
+    print(f"来源: {result.source}")
+    print(f"结果数量: {len(result.data) if result.data else 0}")
+    
+    # 3. 获取联系人
+    print("\n👥 获取联系人列表:")
+    result = await mcp_manager.execute(
+        "contact",
+        "list_contacts",
+        user_id="default"
+    )
+    print(f"来源: {result.source}")
+    print(f"联系人数量: {len(result.data) if result.data else 0}")
+
+
+if __name__ == "__main__":
+    asyncio.run(example_usage())