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/service/comment_service.go

@@ -7,6 +7,7 @@ import (
 
 	"carrot_bbs/internal/cache"
 	"carrot_bbs/internal/model"
+	"carrot_bbs/internal/pkg/cursor"
 	"carrot_bbs/internal/pkg/gorse"
 	"carrot_bbs/internal/pkg/hook"
 	"carrot_bbs/internal/repository"
@@ -355,3 +356,13 @@ func (s *CommentService) Unlike(ctx context.Context, commentID, userID string) e
 func (s *CommentService) IsLiked(ctx context.Context, commentID, userID string) bool {
 	return s.commentRepo.IsLiked(commentID, userID)
 }
+
+// GetCommentsByCursor 游标分页获取帖子评论
+func (s *CommentService) GetCommentsByCursor(ctx context.Context, postID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Comment], error) {
+	return s.commentRepo.GetCommentsByCursor(ctx, postID, req)
+}
+
+// GetRepliesByCursor 游标分页获取根评论的回复
+func (s *CommentService) GetRepliesByCursor(ctx context.Context, rootID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Comment], error) {
+	return s.commentRepo.GetRepliesByCursor(ctx, rootID, req)
+}