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/api/v1/admin.py

@@ -6,7 +6,7 @@ from typing import Optional, List, Dict, Any
 from datetime import datetime, timedelta
 from fastapi import APIRouter, Depends, Query
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy import select, func
+from sqlalchemy import select, func, case
 
 from app.core.deps import get_current_active_user as get_current_user, get_db
 from app.models.user import User
@@ -216,7 +216,7 @@ async def get_course_completion_data(
             select(
                 func.count(UserCourseProgress.id).label('total'),
                 func.sum(
-                    func.case(
+                    case(
                         (UserCourseProgress.status == ProgressStatus.COMPLETED.value, 1),
                         else_=0
                     )