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/studentService.js

@@ -0,0 +1,55 @@
+const Score = require('../models/Score');
+const Student = require('../models/Student');
+
+class StudentService {
+    static async getStudentGrades(userId) {
+        // 先通过 userId 获取 studentId
+        // 假设 users.id = students.id，或者通过 user_id 关联
+        // 根据之前的 authService，我们假设 users.id 就是 students.id
+        
+        // 确认学生是否存在
+        const student = await Student.findById(userId);
+        if (!student) {
+            throw new Error('学生信息不存在');
+        }
+
+        const grades = await Score.findByStudentId(userId);
+
+        // 计算统计信息
+        let totalCredits = 0;
+        let totalGradePoints = 0;
+        const totalCourses = grades.length;
+
+        grades.forEach(grade => {
+            const credit = parseFloat(grade.credit);
+            totalCredits += credit;
+            if (grade.grade_point) {
+                totalGradePoints += parseFloat(grade.grade_point) * credit;
+            }
+        });
+
+        const gpa = totalCredits > 0 ? (totalGradePoints / totalCredits).toFixed(2) : 0;
+        const averageScore = totalCourses > 0 ? 
+            (grades.reduce((sum, g) => sum + parseFloat(g.score || 0), 0) / totalCourses).toFixed(1) : 0;
+
+        return {
+            grades,
+            statistics: {
+                totalCourses,
+                totalCredits,
+                gpa,
+                averageScore
+            }
+        };
+    }
+
+    static async getGradeDetails(scoreId, userId) {
+        const grade = await Score.findDetailsById(scoreId, userId);
+        if (!grade) {
+            throw new Error('成绩不存在');
+        }
+        return grade;
+    }
+}
+
+module.exports = StudentService;