Add assignment regrade update flow

README.md

@@ -25,6 +25,7 @@ docker compose up -d --build
 
 - `GET /api/assignments?username=xxx`
 - `POST /api/assignments` `multipart/form-data`：`username`、`title`、`images`（可多张，按上传顺序）
+- `PUT /api/assignments/{id}` `multipart/form-data`：`username`、`title`、`images`（重新上传并重新批改）
 - `GET /api/assignments/{id}?username=xxx`
 - `DELETE /api/assignments/{id}?username=xxx`