feat(api): add cursor-based pagination for posts, comments, messages, groups, and notifications

Implement cursor-based pagination across multiple API endpoints to improve performance and enable efficient infinite scrolling. This replaces traditional offset-based pagination for high-volume data retrieval.

Changes:
- Add cursor pagination DTOs for all entity types (Post, Comment, Message, Conversation, Group, Notification, GroupMember, GroupAnnouncement)
- Implement cursor pagination methods in repositories with support for various sort orders (created_at, updated_at, join_time, seq)
- Add cursor pagination handlers that maintain backward compatibility with existing offset pagination
- Add new API routes for cursor-based endpoints (/cursor suffix)
- Add helper converter functions for pointer slice types in DTO conversions

internal/handler/notification_handler.go

@@ -5,6 +5,8 @@ import (
 
 	"github.com/gin-gonic/gin"
 
+	"carrot_bbs/internal/dto"
+	"carrot_bbs/internal/pkg/cursor"
 	"carrot_bbs/internal/pkg/response"
 	"carrot_bbs/internal/service"
 )
@@ -130,3 +132,46 @@ func (h *NotificationHandler) ClearAllNotifications(c *gin.Context) {
 
 	response.Success(c, gin.H{"success": true})
 }
+
+// GetNotificationsByCursor 游标分页获取通知列表
+func (h *NotificationHandler) GetNotificationsByCursor(c *gin.Context) {
+	userID := c.GetString("user_id")
+	if userID == "" {
+		response.Unauthorized(c, "")
+		return
+	}
+
+	// 解析游标分页请求
+	req := h.parseCursorRequest(c)
+
+	unreadOnly := c.Query("unread_only") == "true"
+
+	// 调用游标分页服务
+	result, err := h.notificationService.GetNotificationsByCursor(c.Request.Context(), userID, unreadOnly, req)
+	if err != nil {
+		response.InternalServerError(c, "failed to get notifications")
+		return
+	}
+
+	// 转换为响应结构
+	notificationResponses := dto.ConvertNotificationsToResponse(result.Items)
+
+	// 构建游标分页响应
+	cursorResp := &dto.NotificationCursorPageResponse{
+		Items:      notificationResponses,
+		NextCursor: result.NextCursor,
+		PrevCursor: result.PrevCursor,
+		HasMore:    result.HasMore,
+	}
+
+	response.Success(c, cursorResp)
+}
+
+// parseCursorRequest 解析游标分页请求参数
+func (h *NotificationHandler) parseCursorRequest(c *gin.Context) *cursor.PageRequest {
+	cursorStr := c.Query("cursor")
+	direction := cursor.Direction(c.DefaultQuery("direction", "forward"))
+	pageSize, _ := strconv.Atoi(c.DefaultQuery("page_size", "20"))
+
+	return cursor.NewPageRequest(cursorStr, direction, pageSize)
+}