feat: 多班级版 v2.0 - Go后端重写 + 43轮代码审查

- 后端从 Python FastAPI 重写为 Go Gin（端口 56789）
- 多班级完全隔离
- 超级管理员独立登录
- 课代表作业管理、排行榜分项排行
- 角色加减分上下限可配置
- 家长改密功能（可开关）
- 周度/月度重置功能
- MySQL 5.7 兼容
- 43轮代码审查+全部修复
- Apache 2.0 许可证

backend-go/internal/repository/user_repo.go

@@ -0,0 +1,166 @@
+// ===========================================
+// 多班级版班级管理系统 - Go 后端
+//
+// 开发者: Canglan
+// 联系方式: admin@sea-studio.top
+// 版权归属: Sea Network Technology Studio
+// 许可证: Apache License 2.0
+//
+// 版权所有 © Sea Network Technology Studio
+// ===========================================
+
+package repository
+
+import (
+	"time"
+
+	"gorm.io/gorm"
+
+	"hz-gitea.sea-studio.top/canglan/SharedClassManager/internal/model"
+)
+
+// UserRepo 用户数据访问层
+type UserRepo struct {
+	db *gorm.DB
+}
+
+// NewUserRepo 创建用户 Repository
+func NewUserRepo(db *gorm.DB) *UserRepo {
+	return &UserRepo{db: db}
+}
+
+// GetByUsername 根据用户名获取用户（含状态过滤）
+func (r *UserRepo) GetByUsername(username string) (*model.User, error) {
+	var user model.User
+	if err := r.db.Where("username = ? AND status = 1", username).First(&user).Error; err != nil {
+		return nil, err
+	}
+	return &user, nil
+}
+
+// GetByUserID 根据用户ID获取用户
+func (r *UserRepo) GetByUserID(userID int) (*model.User, error) {
+	var user model.User
+	if err := r.db.Where("user_id = ?", userID).First(&user).Error; err != nil {
+		return nil, err
+	}
+	return &user, nil
+}
+
+// CreateStudent 创建学生账号
+func (r *UserRepo) CreateStudent(username, passwordHash, realName string, studentID int) (int, error) {
+	user := model.User{
+		Username:           username,
+		PasswordHash:       passwordHash,
+		RealName:           realName,
+		UserType:           "student",
+		StudentID:          &studentID,
+		Status:             1,
+		NeedChangePassword: 1,
+	}
+	if err := r.db.Create(&user).Error; err != nil {
+		return 0, err
+	}
+	return user.UserID, nil
+}
+
+// CreateParent 创建家长账号
+func (r *UserRepo) CreateParent(username, passwordHash, realName string, studentID int) (int, error) {
+	user := model.User{
+		Username:           username,
+		PasswordHash:       passwordHash,
+		RealName:           realName,
+		UserType:           "parent",
+		StudentID:          &studentID,
+		Status:             1,
+		NeedChangePassword: 0,
+	}
+	if err := r.db.Create(&user).Error; err != nil {
+		return 0, err
+	}
+	return user.UserID, nil
+}
+
+// CreateAdmin 创建管理员账号
+func (r *UserRepo) CreateAdmin(username, passwordHash, realName string) (int, error) {
+	user := model.User{
+		Username:           username,
+		PasswordHash:       passwordHash,
+		RealName:           realName,
+		UserType:           "admin",
+		Status:             1,
+		NeedChangePassword: 1,
+	}
+	if err := r.db.Create(&user).Error; err != nil {
+		return 0, err
+	}
+	return user.UserID, nil
+}
+
+// UpdatePassword 更新密码并清除强制改密标记
+func (r *UserRepo) UpdatePassword(userID int, passwordHash string) error {
+	return r.db.Model(&model.User{}).
+		Where("user_id = ?", userID).
+		Updates(map[string]interface{}{
+			"password_hash":        passwordHash,
+			"need_change_password": 0,
+		}).Error
+}
+
+// UpdateLastLogin 更新最后登录信息
+func (r *UserRepo) UpdateLastLogin(userID int, ip string) error {
+	return r.db.Model(&model.User{}).
+		Where("user_id = ?", userID).
+		Updates(map[string]interface{}{
+			"last_login_time": time.Now(),
+			"last_login_ip":   ip,
+		}).Error
+}
+
+// CheckUsernameExists 检查用户名是否存在
+func (r *UserRepo) CheckUsernameExists(username string) (bool, error) {
+	var count int64
+	if err := r.db.Model(&model.User{}).Where("username = ?", username).Count(&count).Error; err != nil {
+		return false, err
+	}
+	return count > 0, nil
+}
+
+// UpdateStatus 更新用户状态
+func (r *UserRepo) UpdateStatus(userID int, status int8) error {
+	return r.db.Model(&model.User{}).
+		Where("user_id = ?", userID).
+		Update("status", status).Error
+}
+
+// UpdateRealName 更新用户真实姓名
+func (r *UserRepo) UpdateRealName(userID int, realName string) error {
+	return r.db.Model(&model.User{}).
+		Where("user_id = ?", userID).
+		Update("real_name", realName).Error
+}
+
+// GetByStudentID 根据学生ID获取关联的用户账号
+func (r *UserRepo) GetByStudentID(studentID int) (*model.User, error) {
+	var user model.User
+	if err := r.db.Where("student_id = ? AND status = 1", studentID).First(&user).Error; err != nil {
+		return nil, err
+	}
+	return &user, nil
+}
+
+// DeleteUser 硬删除用户记录
+func (r *UserRepo) DeleteUser(userID int) error {
+	return r.db.Unscoped().Where("user_id = ?", userID).Delete(&model.User{}).Error
+}
+
+// GetActiveUsernames 获取所有活跃用户的用户名列表（用于批量导入去重）
+func (r *UserRepo) GetActiveUsernames() ([]string, error) {
+	var usernames []string
+	if err := r.db.Model(&model.User{}).
+		Where("status = 1").
+		Pluck("username", &usernames).Error; err != nil {
+		return nil, err
+	}
+	return usernames, nil
+}