refactor: 后端代码优化 11 项（codestable/refactors/2026-04-25-backend-cleanup）

- 提取 getWKFromContext 辅助函数，消除 handler 中 5 处重复代码
- 提取 retryCode 函数，消除 Login/performStudy 中验证码重试重复
- 提取 removeSession 内部方法，消除 Del/ClearAll/ClearExpired 中 3 处重复
- 提取 WK.UserKey() 方法，消除 4 处 userKey 手动拼接
- SessionManager.Get() 改用 RLock 优化读性能
- GetRecords 递归分页改为迭代，避免栈溢出
- prepareRequestClient 添加配置缓存，仅在 debug 设置变化时重建
- 修正 schedule.go 时区为 Asia/Shanghai + cron "0 6 * * *"
- 修正 typo "以达到" → "已达到"
- 删除未使用的 QAList struct
- 修复 bufferHub.append 切片内存泄漏

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/ckwk/types.go

@@ -25,7 +25,7 @@ const (
 
 	StudyStart StudyStatus = 1 // 开始学习
 	Study      StudyStatus = 2 // 学习中
-	StudyOver  StudyStatus = 3 // 学习介绍
+	StudyOver  StudyStatus = 3 // 学习结束
 )
 
 // User: 用户
@@ -49,11 +49,36 @@ type Course struct {
 	Type      string  `json:"type"`
 }
 
-type QAList struct {
-}
-
+// GetRecords 分页获取记录（迭代实现）
 func GetRecords[T any](wk *WK, rType RecordType, courseID, page string) (*AllRecordResp[T], error) {
 	log.Debug("获取记录信息", zap.String("host", wk.Host), zap.String("type", string(rType)))
+
+	result, err := fetchRecordPage[T](wk, rType, courseID, page)
+	if err != nil {
+		return nil, err
+	}
+	if !result.Status {
+		return &result, fmt.Errorf("接口报错: %s", result.Msg)
+	}
+
+	for result.PageInfo.Page < result.PageInfo.PageCount {
+		nextPage := fmt.Sprint(result.PageInfo.Page + 1)
+		nextResult, err := fetchRecordPage[T](wk, rType, courseID, nextPage)
+		if err != nil {
+			return nil, err
+		}
+
+		result.List = append(result.List, nextResult.List...)
+		log.Debug("Page", zap.Int("currentPage", result.PageInfo.Page), zap.Int("nextPage", nextResult.PageInfo.Page))
+		result.PageInfo = nextResult.PageInfo
+	}
+
+	return &result, nil
+}
+
+// fetchRecordPage 获取单页记录
+func fetchRecordPage[T any](wk *WK, rType RecordType, courseID, page string) (AllRecordResp[T], error) {
+	var result AllRecordResp[T]
 	resp, err := wk.newRequest().
 		SetQueryParams(map[string]string{
 			"courseId": courseID,
@@ -62,32 +87,13 @@ func GetRecords[T any](wk *WK, rType RecordType, courseID, page string) (*AllRec
 		}).
 		Get(fmt.Sprintf("https://%s/user/study_record%s.json", wk.Host, rType))
 	if err != nil {
-		return nil, err
+		return result, err
 	}
 
-	var result AllRecordResp[T]
 	if err := json.Unmarshal(resp.Bytes(), &result); err != nil {
 		log.Error("JSON解析失败", zap.Error(err))
-		return nil, err
+		return result, err
 	}
 
-	if !result.Status {
-		return &result, fmt.Errorf("接口报错: %s", result.Msg)
-	}
-	currentPage := result.PageInfo.Page
-	totalPages := result.PageInfo.PageCount
-
-	for currentPage < totalPages {
-		nextPage := fmt.Sprint(currentPage + 1)
-		nextResult, err := GetRecords[T](wk, rType, courseID, nextPage)
-		if err != nil {
-			return nil, err
-		}
-
-		result.List = append(result.List, nextResult.List...)
-		log.Debug("Page", zap.Int("currentPage", currentPage), zap.Int("Page", nextResult.PageInfo.Page))
-		currentPage = nextResult.PageInfo.Page
-	}
-
-	return &result, nil
+	return result, nil
 }