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/router/router.go

@@ -263,12 +263,14 @@ func (r *Router) setupRoutes() {
 		comments := v1.Group("/comments")
 		{
 			comments.GET("/post/:id", middleware.OptionalAuth(r.jwtService), r.commentHandler.GetByPostID)
+			comments.GET("/post/:id/cursor", middleware.OptionalAuth(r.jwtService), r.commentHandler.GetByPostIDByCursor) // 帖子评论游标分页
 			comments.GET("/:id", middleware.OptionalAuth(r.jwtService), r.commentHandler.GetByID)
 			comments.POST("", authMiddleware, r.commentHandler.Create)
 			comments.PUT("/:id", authMiddleware, r.commentHandler.Update)
 			comments.DELETE("/:id", authMiddleware, r.commentHandler.Delete)
 			comments.GET("/:id/replies", middleware.OptionalAuth(r.jwtService), r.commentHandler.GetReplies)
-			comments.GET("/:id/replies/flat", middleware.OptionalAuth(r.jwtService), r.commentHandler.GetRepliesByRootID) // 扁平化分页获取回复
+			comments.GET("/:id/replies/flat", middleware.OptionalAuth(r.jwtService), r.commentHandler.GetRepliesByRootID)           // 扁平化分页获取回复
+			comments.GET("/:id/replies/cursor", middleware.OptionalAuth(r.jwtService), r.commentHandler.GetRepliesByRootIDByCursor) // 回复游标分页
 			// 评论点赞
 			comments.POST("/:id/like", authMiddleware, r.commentHandler.Like)
 			comments.DELETE("/:id/like", authMiddleware, r.commentHandler.Unlike)
@@ -282,9 +284,11 @@ func (r *Router) setupRoutes() {
 			// 新的 RESTful 风格路由（推荐使用）
 			// ================================================================
 			conversations.GET("", r.messageHandler.HandleGetConversationList)                   // 列表
+			conversations.GET("/cursor", r.messageHandler.GetConversationsByCursor)             // 会话列表游标分页
 			conversations.POST("", r.messageHandler.HandleCreateConversation)                   // 创建
 			conversations.GET("/:id", r.messageHandler.HandleGetConversation)                   // 详情
 			conversations.GET("/:id/messages", r.messageHandler.HandleGetMessages)              // 消息列表
+			conversations.GET("/:id/messages/cursor", r.messageHandler.GetMessagesByCursor)     // 消息列表游标分页
 			conversations.POST("/:id/messages", r.messageHandler.HandleSendMessage)             // 发送消息
 			conversations.POST("/:id/read", r.messageHandler.HandleMarkRead)                    // 标记已读
 			conversations.PUT("/:id/pinned", r.messageHandler.HandleSetConversationPinned)      // 置顶设置
@@ -312,6 +316,7 @@ func (r *Router) setupRoutes() {
 		notifications := v1.Group("/notifications")
 		{
 			notifications.GET("", authMiddleware, r.notificationHandler.GetNotifications)
+			notifications.GET("/cursor", authMiddleware, r.notificationHandler.GetNotificationsByCursor) // 通知游标分页
 			notifications.POST("/:id/read", authMiddleware, r.notificationHandler.MarkAsRead)
 			notifications.POST("/read-all", authMiddleware, r.notificationHandler.MarkAllAsRead)
 			notifications.GET("/unread-count", authMiddleware, r.notificationHandler.GetUnreadCount)
@@ -360,6 +365,7 @@ func (r *Router) setupRoutes() {
 				// ================================================================
 				// 群组基本操作
 				groups.GET("", r.groupHandler.HandleGetUserGroups)               // 列表
+				groups.GET("/cursor", r.groupHandler.GetGroupsByCursor)          // 群组游标分页
 				groups.POST("", r.groupHandler.HandleCreateGroup)                // 创建
 				groups.GET("/:id", r.groupHandler.HandleGetGroupInfo)            // 详情
 				groups.GET("/:id/me", r.groupHandler.HandleGetMyMemberInfo)      // 当前用户成员信息
@@ -372,6 +378,7 @@ func (r *Router) setupRoutes() {
 				groups.POST("/:id/join-requests/respond", r.groupHandler.HandleRespondInvite) // 响应加群邀请/申请
 				groups.POST("/:id/leave", r.groupHandler.HandleSetGroupLeave)                 // 退群
 				groups.GET("/:id/members", r.groupHandler.HandleGetGroupMemberList)           // 成员列表
+				groups.GET("/:id/members/cursor", r.groupHandler.GetMembersByCursor)          // 成员游标分页
 				groups.POST("/:id/members/kick", r.groupHandler.HandleSetGroupKick)           // 踢出成员
 				groups.PUT("/:id/members/:user_id/admin", r.groupHandler.HandleSetGroupAdmin) // 设置管理员
 				groups.PUT("/:id/members/me/nickname", r.groupHandler.HandleSetNickname)      // 设置群昵称
@@ -386,6 +393,7 @@ func (r *Router) setupRoutes() {
 				// 群公告
 				groups.POST("/:id/announcements", r.groupHandler.HandleCreateAnnouncement)                    // 创建公告
 				groups.GET("/:id/announcements", r.groupHandler.HandleGetAnnouncements)                       // 公告列表
+				groups.GET("/:id/announcements/cursor", r.groupHandler.GetAnnouncementsByCursor)              // 公告游标分页
 				groups.DELETE("/:id/announcements/:announcement_id", r.groupHandler.HandleDeleteAnnouncement) // 删除公告
 
 				// 加群请求处理