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/__init__.py

@@ -0,0 +1,21 @@
+"""
+MCP (Model Context Protocol) 集成模块
+统一外部接口管理层
+"""
+from .mcp_manager import MCPManager, mcp_manager
+from .mcp_client import MCPClient, MCPServerConfig
+from .adapters.base_adapter import BaseAdapter, AdapterResult
+from .adapters import ContactAdapter, DictionaryAdapter, NewsAdapter
+
+__all__ = [
+    "MCPManager", 
+    "mcp_manager", 
+    "MCPClient", 
+    "MCPServerConfig",
+    "BaseAdapter", 
+    "AdapterResult",
+    "ContactAdapter", 
+    "DictionaryAdapter", 
+    "NewsAdapter"
+]
+