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/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
+}