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/ranking_service.go

@@ -0,0 +1,80 @@
+// ===========================================
+// 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 (
+	"hz-gitea.sea-studio.top/canglan/SharedClassManager/internal/repository"
+)
+
+// RankingService 排行榜服务
+type RankingService struct {
+	studentRepo *repository.StudentRepo
+	conductRepo *repository.ConductRepo
+}
+
+// NewRankingService 创建排行榜服务
+func NewRankingService(
+	studentRepo *repository.StudentRepo,
+	conductRepo *repository.ConductRepo,
+) *RankingService {
+	return &RankingService{
+		studentRepo: studentRepo,
+		conductRepo: conductRepo,
+	}
+}
+
+// GetRankings 获取排行榜
+func (s *RankingService) GetRankings(classID int, rankType string, limit int) (map[string]interface{}, error) {
+	switch rankType {
+	case "attendance", "homework", "conduct":
+		return s.getTypedRanking(classID, rankType, limit)
+	default:
+		// 默认按操行分总分排行
+		ranking, err := s.studentRepo.GetRanking(classID, limit)
+		if err != nil {
+			return nil, err
+		}
+		totalStudents, _ := s.studentRepo.GetTotalCount(classID)
+		return map[string]interface{}{
+			"ranking":        ranking,
+			"total_students": totalStudents,
+			"type":           "all",
+		}, nil
+	}
+}
+
+// getTypedRanking 获取分项排行榜（使用 SQL 层聚合，避免全量加载）
+func (s *RankingService) getTypedRanking(classID int, relatedType string, limit int) (map[string]interface{}, error) {
+	dbType := relatedType
+	if relatedType == "conduct" {
+		dbType = "manual"
+	}
+	results, err := s.conductRepo.GetStudentPointsByType(classID, dbType, limit)
+	if err != nil {
+		return nil, err
+	}
+
+	var rankings []map[string]interface{}
+	for _, r := range results {
+		rankings = append(rankings, map[string]interface{}{
+			"student_id": r.StudentID,
+			"student_no": r.StudentNo,
+			"name":       r.Name,
+			"points":     r.TotalPoints,
+		})
+	}
+
+	return map[string]interface{}{
+		"ranking": rankings,
+		"type":    relatedType,
+	}, nil
+}