修复前后端显示异常问题（受试者、中心、负责人、日期等）

backend/app/schemas/task.py

@@ -4,6 +4,8 @@ from typing import Optional
 
 from pydantic import BaseModel, ConfigDict
 
+from app.schemas.user import UserDisplay
+
 
 class TaskCreate(BaseModel):
     milestone_id: Optional[uuid.UUID] = None
@@ -31,6 +33,7 @@ class TaskRead(BaseModel):
     title: str
     description: Optional[str]
     assignee_id: Optional[uuid.UUID]
+    assignee: Optional[UserDisplay] = None
     priority: str
     due_date: Optional[date]
     status: str