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/dto/dto.go

@@ -1179,3 +1179,92 @@ type PendingContentAuthor struct {
 type OnlineUsersResponse struct {
 	Count int64 `json:"count"` // 在线用户数
 }
+
+// ==================== Cursor Pagination DTOs ====================
+
+// CursorPageRequest 游标分页请求
+type CursorPageRequest struct {
+	Cursor    string `json:"cursor" form:"cursor"`       // 游标（首次请求为空）
+	Direction string `json:"direction" form:"direction"` // forward | backward
+	PageSize  int    `json:"page_size" form:"page_size"` // 每页数量
+}
+
+// CursorPageResponse 游标分页响应（泛型版本）
+// 注意：由于 Go 的限制，这里使用 interface{} 作为 Items 类型
+// 实际使用时可以创建特定类型的响应结构体
+type CursorPageResponse struct {
+	Items      interface{} `json:"items"`
+	NextCursor string      `json:"next_cursor,omitempty"` // 下一页游标
+	PrevCursor string      `json:"prev_cursor,omitempty"` // 上一页游标（可选）
+	HasMore    bool        `json:"has_more"`              // 是否有更多数据
+}
+
+// CursorPageMeta 分页元信息
+type CursorPageMeta struct {
+	PageSize  int    `json:"page_size"`
+	Direction string `json:"direction"`
+}
+
+// PostCursorPageResponse 帖子游标分页响应
+type PostCursorPageResponse struct {
+	Items      []*PostResponse `json:"items"`
+	NextCursor string          `json:"next_cursor,omitempty"`
+	PrevCursor string          `json:"prev_cursor,omitempty"`
+	HasMore    bool            `json:"has_more"`
+}
+
+// CommentCursorPageResponse 评论游标分页响应
+type CommentCursorPageResponse struct {
+	Items      []*CommentResponse `json:"items"`
+	NextCursor string             `json:"next_cursor,omitempty"`
+	PrevCursor string             `json:"prev_cursor,omitempty"`
+	HasMore    bool               `json:"has_more"`
+}
+
+// MessageCursorPageResponse 消息游标分页响应
+type MessageCursorPageResponse struct {
+	Items      []*MessageResponse `json:"items"`
+	NextCursor string             `json:"next_cursor,omitempty"`
+	PrevCursor string             `json:"prev_cursor,omitempty"`
+	HasMore    bool               `json:"has_more"`
+}
+
+// ConversationCursorPageResponse 会话游标分页响应
+type ConversationCursorPageResponse struct {
+	Items      []*ConversationResponse `json:"items"`
+	NextCursor string                  `json:"next_cursor,omitempty"`
+	PrevCursor string                  `json:"prev_cursor,omitempty"`
+	HasMore    bool                    `json:"has_more"`
+}
+
+// GroupCursorPageResponse 群组游标分页响应
+type GroupCursorPageResponse struct {
+	Items      []*GroupResponse `json:"items"`
+	NextCursor string           `json:"next_cursor,omitempty"`
+	PrevCursor string           `json:"prev_cursor,omitempty"`
+	HasMore    bool             `json:"has_more"`
+}
+
+// NotificationCursorPageResponse 通知游标分页响应
+type NotificationCursorPageResponse struct {
+	Items      []*NotificationResponse `json:"items"`
+	NextCursor string                  `json:"next_cursor,omitempty"`
+	PrevCursor string                  `json:"prev_cursor,omitempty"`
+	HasMore    bool                    `json:"has_more"`
+}
+
+// GroupMemberCursorPageResponse 群成员游标分页响应
+type GroupMemberCursorPageResponse struct {
+	Items      []*GroupMemberResponse `json:"items"`
+	NextCursor string                 `json:"next_cursor,omitempty"`
+	PrevCursor string                 `json:"prev_cursor,omitempty"`
+	HasMore    bool                   `json:"has_more"`
+}
+
+// GroupAnnouncementCursorPageResponse 群公告游标分页响应
+type GroupAnnouncementCursorPageResponse struct {
+	Items      []*GroupAnnouncementResponse `json:"items"`
+	NextCursor string                       `json:"next_cursor,omitempty"`
+	PrevCursor string                       `json:"prev_cursor,omitempty"`
+	HasMore    bool                         `json:"has_more"`
+}

internal/dto/group_converter.go

@@ -94,3 +94,30 @@ func GroupAnnouncementsToResponse(announcements []model.GroupAnnouncement) []*Gr
 	}
 	return result
 }
+
+// GroupsToResponseFromPtr 将Group指针切片转换为GroupResponse列表
+func GroupsToResponseFromPtr(groups []*model.Group) []*GroupResponse {
+	result := make([]*GroupResponse, 0, len(groups))
+	for _, group := range groups {
+		result = append(result, GroupToResponse(group))
+	}
+	return result
+}
+
+// GroupMembersToResponseFromPtr 将GroupMember指针切片转换为GroupMemberResponse列表
+func GroupMembersToResponseFromPtr(members []*model.GroupMember) []*GroupMemberResponse {
+	result := make([]*GroupMemberResponse, 0, len(members))
+	for _, member := range members {
+		result = append(result, GroupMemberToResponse(member))
+	}
+	return result
+}
+
+// GroupAnnouncementsToResponseFromPtr 将GroupAnnouncement指针切片转换为GroupAnnouncementResponse列表
+func GroupAnnouncementsToResponseFromPtr(announcements []*model.GroupAnnouncement) []*GroupAnnouncementResponse {
+	result := make([]*GroupAnnouncementResponse, 0, len(announcements))
+	for _, announcement := range announcements {
+		result = append(result, GroupAnnouncementToResponse(announcement))
+	}
+	return result
+}