feat: Implement file download token management and restructure file routes

- Added DownloadTokenManager for creating and verifying JWT download tokens.
- Introduced new download routes for creating download tokens and downloading files using tokens.
- Restructured file upload routes into a dedicated sub-router.
- Updated file upload session management with improved error handling and response structures.
- Created a new MCP (Microservice Communication Protocol) router with basic request and response models.
- Added base models for MCP requests and responses, including method enumeration.

README.md

@@ -171,7 +171,7 @@ fastapi dev
 fastapi run
 ```
 
-访问 http://localhost:8000/docs 查看 API 文档。
+访问 <http://localhost:8000/docs> 查看 API 文档。
 
 ## 测试