fix: 修复 SQLAlchemy case() 语法

- admin.py: 使用 case() 替代 func.case()
- task_service.py: 同上
- 已在服务器执行数据库迁移创建 user_course_progress 表

backend/app/services/task_service.py

@@ -3,7 +3,7 @@
 """
 from typing import List, Optional
 from datetime import datetime
-from sqlalchemy import select, func, and_
+from sqlalchemy import select, func, and_, case
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import joinedload
 from app.models.task import Task, TaskCourse, TaskAssignment, TaskStatus, AssignmentStatus
@@ -154,7 +154,7 @@ class TaskService(BaseService[Task]):
         stmt = select(
             func.count(TaskAssignment.id).label('total'),
             func.sum(
-                func.case(
+                case(
                     (TaskAssignment.status == AssignmentStatus.COMPLETED, 1),
                     else_=0
                 )