refactor(notification): migrate cursor pagination to use unified cursor package

- Add new GetByReceiverIDCursor method with context support and generic cursor types
- Rename old implementation to GetByReceiverIDCursorLegacy for backward compatibility
- Update handler to use legacy interface temporarily during migration
- Use CursorBuilder pattern for consistent pagination across services

internal/repository/system_notification_repo.go

@@ -1,7 +1,11 @@
 package repository
 
 import (
+	"context"
+	"strconv"
+
 	"carrot_bbs/internal/model"
+	"carrot_bbs/internal/pkg/cursor"
 
 	"gorm.io/gorm"
 )
@@ -114,13 +118,67 @@ func (r *SystemNotificationRepository) GetByType(receiverID string, notifyType m
 }
 
 // GetByReceiverIDCursor 游标分页获取用户的通知列表
-func (r *SystemNotificationRepository) GetByReceiverIDCursor(receiverID string, cursor string, pageSize int) ([]*model.SystemNotification, string, bool, error) {
+// 使用统一的 cursor 包进行游标分页
+func (r *SystemNotificationRepository) GetByReceiverIDCursor(ctx context.Context, receiverID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.SystemNotification], error) {
+	// 构建基础查询
+	query := r.db.WithContext(ctx).Model(&model.SystemNotification{}).Where("receiver_id = ?", receiverID)
+
+	// 使用 CursorBuilder 构建游标查询
+	builder := cursor.NewBuilder(query, cursor.SortByCreatedAtDesc).
+		WithCursor(req.Cursor, cursor.Direction(req.Direction)).
+		WithPageSize(req.PageSize)
+
+	if builder.Error() != nil {
+		return nil, builder.Error()
+	}
+
+	// 执行查询
+	var notifications []*model.SystemNotification
+	if err := builder.Build().Find(&notifications).Error; err != nil {
+		return nil, err
+	}
+
+	// 构建响应
+	pageSize := builder.GetPageSize()
+	hasMore := cursor.HasMore(len(notifications), pageSize)
+	if hasMore {
+		notifications = notifications[:pageSize]
+	}
+
+	// 生成下一页游标
+	var nextCursor string
+	if hasMore && len(notifications) > 0 {
+		lastNotification := notifications[len(notifications)-1]
+		nextCursor = cursor.NewCursor(
+			cursor.FormatTime(lastNotification.CreatedAt),
+			strconv.FormatInt(lastNotification.ID, 10),
+			cursor.SortByCreatedAtDesc,
+		).Encode()
+	}
+
+	return &cursor.CursorPageResult[*model.SystemNotification]{
+		Items:      notifications,
+		NextCursor: nextCursor,
+		HasMore:    hasMore,
+	}, nil
+}
+
+// GetByReceiverIDCursorLegacy 游标分页获取用户的通知列表（旧版兼容接口）
+// 已废弃：请使用 GetByReceiverIDCursor
+func (r *SystemNotificationRepository) GetByReceiverIDCursorLegacy(receiverID string, cursorStr string, pageSize int) ([]*model.SystemNotification, string, bool, error) {
+	// 构建基础查询
 	query := r.db.Model(&model.SystemNotification{}).Where("receiver_id = ?", receiverID)
 
 	// 如果有游标，解析游标并添加条件
-	if cursor != "" {
-		// 游标格式：createdAt_id
-		query = query.Where("created_at < ? OR (created_at = ? AND id < ?)", cursor, cursor, cursor)
+	if cursorStr != "" {
+		decodedCursor, err := cursor.DecodeCursor(cursorStr)
+		if err != nil {
+			// 游标格式无效，返回空结果
+			return []*model.SystemNotification{}, "", false, nil
+		}
+		// 使用元组比较进行分页
+		condition := "(created_at, id) < (?, ?)"
+		query = query.Where(condition, decodedCursor.SortValue, decodedCursor.ID)
 	}
 
 	// 多取一条用于判断是否还有下一页
@@ -142,7 +200,11 @@ func (r *SystemNotificationRepository) GetByReceiverIDCursor(receiverID string,
 	var nextCursor string
 	if hasMore && len(notifications) > 0 {
 		lastNotification := notifications[len(notifications)-1]
-		nextCursor = lastNotification.CreatedAt.Format("2006-01-02 15:04:05.999999999")
+		nextCursor = cursor.NewCursor(
+			cursor.FormatTime(lastNotification.CreatedAt),
+			strconv.FormatInt(lastNotification.ID, 10),
+			cursor.SortByCreatedAtDesc,
+		).Encode()
 	}
 
 	return notifications, nextCursor, hasMore, nil