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/service/subject_service.go

@@ -0,0 +1,92 @@
+// ===========================================
+// 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 service
+
+import (
+	"fmt"
+
+	"hz-gitea.sea-studio.top/canglan/SharedClassManager/internal/model"
+	"hz-gitea.sea-studio.top/canglan/SharedClassManager/internal/repository"
+	"hz-gitea.sea-studio.top/canglan/SharedClassManager/pkg/logger"
+)
+
+// SubjectService 科目服务
+type SubjectService struct {
+	subjectRepo *repository.SubjectRepo
+}
+
+// NewSubjectService 创建科目服务
+func NewSubjectService(subjectRepo *repository.SubjectRepo) *SubjectService {
+	return &SubjectService{subjectRepo: subjectRepo}
+}
+
+// GetSubjects 获取科目列表
+func (s *SubjectService) GetSubjects(isActive *bool) (map[string]interface{}, error) {
+	subjects, err := s.subjectRepo.GetAll(isActive)
+	if err != nil {
+		return nil, err
+	}
+	return map[string]interface{}{
+		"subjects": subjects,
+		"total":    len(subjects),
+	}, nil
+}
+
+// CreateSubject 创建科目
+func (s *SubjectService) CreateSubject(subjectName string, subjectCode *string, sortOrder int) (map[string]interface{}, error) {
+	existing, _ := s.subjectRepo.GetByName(subjectName)
+	if existing != nil {
+		return map[string]interface{}{"success": false, "message": "科目名称已存在"}, nil
+	}
+
+	subject := &model.Subject{
+		SubjectName: subjectName,
+		SubjectCode: subjectCode,
+		SortOrder:   sortOrder,
+		IsActive:    1,
+	}
+
+	subjectID, err := s.subjectRepo.Create(subject)
+	if err != nil {
+		return nil, err
+	}
+
+	logger.Sugared.Infof("创建科目: %s", subjectName)
+	return map[string]interface{}{
+		"success":     true,
+		"subject_id":  subjectID,
+	}, nil
+}
+
+// UpdateSubject 更新科目
+func (s *SubjectService) UpdateSubject(subjectID int, updates map[string]interface{}) error {
+	return s.subjectRepo.Update(subjectID, updates)
+}
+
+// DisableSubject 禁用科目（将 is_active 设为 0，保留数据）
+func (s *SubjectService) DisableSubject(subjectID int) error {
+	return s.subjectRepo.Update(subjectID, map[string]interface{}{"is_active": 0})
+}
+
+// EnableSubject 启用科目（将 is_active 设为 1）
+func (s *SubjectService) EnableSubject(subjectID int) error {
+	return s.subjectRepo.Update(subjectID, map[string]interface{}{"is_active": 1})
+}
+
+// DeleteSubject 物理删除科目（需先检查关联数据）
+func (s *SubjectService) DeleteSubject(subjectID int) error {
+	hasData, _ := s.subjectRepo.HasRelatedData(subjectID)
+	if hasData {
+		return fmt.Errorf("该科目下已有作业数据，无法删除")
+	}
+	return s.subjectRepo.Delete(subjectID)
+}