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/subject_repo.go

@@ -0,0 +1,104 @@
+// ===========================================
+// 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 (
+	"gorm.io/gorm"
+
+	"hz-gitea.sea-studio.top/canglan/SharedClassManager/internal/model"
+)
+
+// SubjectRepo 科目数据访问层
+type SubjectRepo struct {
+	db *gorm.DB
+}
+
+// NewSubjectRepo 创建科目 Repository
+func NewSubjectRepo(db *gorm.DB) *SubjectRepo {
+	return &SubjectRepo{db: db}
+}
+
+// GetAll 获取所有科目列表
+func (r *SubjectRepo) GetAll(isActive *bool) ([]model.Subject, error) {
+	var subjects []model.Subject
+	query := r.db.Where("1 = 1")
+	if isActive != nil {
+		if *isActive {
+			query = query.Where("is_active = 1")
+		} else {
+			query = query.Where("is_active = 0")
+		}
+	}
+	if err := query.Order("sort_order, subject_id").Find(&subjects).Error; err != nil {
+		return nil, err
+	}
+	return subjects, nil
+}
+
+// GetByID 根据ID获取科目
+func (r *SubjectRepo) GetByID(subjectID int) (*model.Subject, error) {
+	var subject model.Subject
+	if err := r.db.Where("subject_id = ?", subjectID).First(&subject).Error; err != nil {
+		return nil, err
+	}
+	return &subject, nil
+}
+
+// GetByName 根据科目名称获取科目
+func (r *SubjectRepo) GetByName(subjectName string) (*model.Subject, error) {
+	var subject model.Subject
+	if err := r.db.Where("subject_name = ?", subjectName).First(&subject).Error; err != nil {
+		return nil, err
+	}
+	return &subject, nil
+}
+
+// Create 创建科目
+func (r *SubjectRepo) Create(subject *model.Subject) (int, error) {
+	if err := r.db.Create(subject).Error; err != nil {
+		return 0, err
+	}
+	return subject.SubjectID, nil
+}
+
+// Update 更新科目信息
+func (r *SubjectRepo) Update(subjectID int, updates map[string]interface{}) error {
+	if len(updates) == 0 {
+		return nil
+	}
+	return r.db.Model(&model.Subject{}).
+		Where("subject_id = ?", subjectID).
+		Updates(updates).Error
+}
+
+// Delete 删除科目
+func (r *SubjectRepo) Delete(subjectID int) error {
+	return r.db.Where("subject_id = ?", subjectID).Delete(&model.Subject{}).Error
+}
+
+// HasRelatedData 检查科目是否有关联的作业数据
+func (r *SubjectRepo) HasRelatedData(subjectID int) (bool, error) {
+	var count int64
+	if err := r.db.Model(&model.Assignment{}).
+		Where("subject_id = ?", subjectID).
+		Count(&count).Error; err != nil {
+		return false, err
+	}
+	return count > 0, nil
+}
+
+// Activate 激活科目
+func (r *SubjectRepo) Activate(subjectID int) error {
+	return r.db.Model(&model.Subject{}).
+		Where("subject_id = ?", subjectID).
+		Update("is_active", 1).Error
+}