fix: 修复任务API枚举值访问错误

- 使用 hasattr 检查是否为枚举类型，兼容字符串和枚举
- 移除 get_tasks 中多余的 get_task_detail 调用，使用已加载的关联数据

backend/app/api/v1/tasks.py

@@ -43,13 +43,20 @@ async def create_task(
     
     # 构建响应
     courses = [link.course.name for link in task.course_links]
+    # 安全获取枚举值（兼容字符串和枚举类型）
+    priority_val = task.priority.value if hasattr(task.priority, 'value') else task.priority
+    status_val = task.status.value if hasattr(task.status, 'value') else task.status
+    completed_count = sum(
+        1 for a in task.assignments 
+        if (a.status.value if hasattr(a.status, 'value') else a.status) == "completed"
+    )
     return ResponseModel(
         data=TaskResponse(
             id=task.id,
             title=task.title,
             description=task.description,
-            priority=task.priority.value,
-            status=task.status.value,
+            priority=priority_val,
+            status=status_val,
             creator_id=task.creator_id,
             deadline=task.deadline,
             requirements=task.requirements,
@@ -58,7 +65,7 @@ async def create_task(
             updated_at=task.updated_at,
             courses=courses,
             assigned_count=len(task.assignments),
-            completed_count=sum(1 for a in task.assignments if a.status.value == "completed")
+            completed_count=completed_count
         )
     )
 
@@ -77,26 +84,33 @@ async def get_tasks(
     # 构建响应
     items = []
     for task in tasks:
-        # 加载关联数据
-        task_detail = await task_service.get_task_detail(db, task.id)
-        if task_detail:
-            courses = [link.course.name for link in task_detail.course_links]
-            items.append(TaskResponse(
-                id=task.id,
-                title=task.title,
-                description=task.description,
-                priority=task.priority.value,
-                status=task.status.value,
-                creator_id=task.creator_id,
-                deadline=task.deadline,
-                requirements=task.requirements,
-                progress=task.progress,
-                created_at=task.created_at,
-                updated_at=task.updated_at,
-                courses=courses,
-                assigned_count=len(task_detail.assignments),
-                completed_count=sum(1 for a in task_detail.assignments if a.status.value == "completed")
-            ))
+        # 安全获取枚举值
+        priority_val = task.priority.value if hasattr(task.priority, 'value') else task.priority
+        status_val = task.status.value if hasattr(task.status, 'value') else task.status
+        
+        # 使用已加载的关联数据（通过 selectinload）
+        courses = [link.course.name for link in task.course_links] if task.course_links else []
+        completed_count = sum(
+            1 for a in task.assignments 
+            if (a.status.value if hasattr(a.status, 'value') else a.status) == "completed"
+        ) if task.assignments else 0
+        
+        items.append(TaskResponse(
+            id=task.id,
+            title=task.title,
+            description=task.description,
+            priority=priority_val,
+            status=status_val,
+            creator_id=task.creator_id,
+            deadline=task.deadline,
+            requirements=task.requirements,
+            progress=task.progress,
+            created_at=task.created_at,
+            updated_at=task.updated_at,
+            courses=courses,
+            assigned_count=len(task.assignments) if task.assignments else 0,
+            completed_count=completed_count
+        ))
     
     return ResponseModel(
         data=PaginatedResponse.create(