修复数据库连接错误，用户组功能待测试

api/models/examTask.ts

@@ -417,7 +417,7 @@ export class ExamTaskModel {
     let totalScore = questions.reduce((sum, q) => sum + q.score, 0);
     while (totalScore < subject.totalScore) {
       // 获取所有类型的随机题目
-      const allTypes = Object.keys(subject.typeRatios).filter(type => subject.typeRatios[type] > 0);
+      const allTypes = Object.keys(subject.typeRatios).filter(type => subject.typeRatios[type as keyof typeof subject.typeRatios] > 0);
       if (allTypes.length === 0) break;
       
       const randomType = allTypes[Math.floor(Math.random() * allTypes.length)];