feat: 实现成绩管理系统核心功能

添加响应工具、错误处理中间件和数据库模型
创建用户、学生、课程和成绩相关服务
实现管理员、教师和学生控制器的基本功能
重构路由处理并优化数据库查询

backend/services/teacherService.js

@@ -0,0 +1,50 @@
+const Course = require('../models/Course');
+const Score = require('../models/Score');
+const Student = require('../models/Student');
+
+class TeacherService {
+    static async getCourses(teacherId) {
+        return await Course.findByTeacherId(teacherId);
+    }
+
+    static async addScore(teacherId, scoreData) {
+        const { studentId, courseId, score } = scoreData;
+
+        // 验证学生
+        const student = await Student.findById(studentId);
+        if (!student) {
+            throw new Error('学生不存在');
+        }
+
+        // 验证课程（可选：验证是否是该教师的课程）
+        // const course = await Course.findById(courseId);
+        
+        // 检查重复
+        const existingScore = await Score.findByStudentAndCourse(studentId, courseId);
+        if (existingScore) {
+            throw new Error('该学生此课程成绩已存在');
+        }
+
+        // 计算绩点和等级
+        const numericScore = parseFloat(score);
+        let gradePoint = 0;
+        let gradeLevel = 'F';
+
+        if (numericScore >= 90) { gradePoint = 4.0; gradeLevel = 'A'; }
+        else if (numericScore >= 80) { gradePoint = 3.0; gradeLevel = 'B'; }
+        else if (numericScore >= 70) { gradePoint = 2.0; gradeLevel = 'C'; }
+        else if (numericScore >= 60) { gradePoint = 1.0; gradeLevel = 'D'; }
+
+        const fullScoreData = {
+            ...scoreData,
+            teacherId,
+            gradePoint,
+            gradeLevel
+        };
+
+        const gradeId = await Score.create(fullScoreData);
+        return gradeId;
+    }
+}
+
+module.exports = TeacherService;