feat(posts): add preview URL fields for post images

Add PreviewURL and PreviewURLLarge fields to PostImage model and responses. Image data now supports format "url|preview_url|preview_url_large" for storing multiple image variants. Also change comment cursor pagination to sort by created_at ASC (oldest first) to display earliest comments as first floor, and make AdminReviewVerificationRequest.Approve field optional.

internal/repository/comment_repo.go

@@ -495,14 +495,14 @@ func (r *commentRepository) GetCommentsByPostIDForAdmin(postID string, page, pag
 // ========== Cursor Pagination Methods ==========
 
 // GetCommentsByCursor 游标分页获取帖子评论（包含回复）
-// 排序方式：created_at DESC
+// 排序方式：created_at ASC（最早发布的为一楼）
 func (r *commentRepository) GetCommentsByCursor(ctx context.Context, postID string, req *cursor.PageRequest) (*cursor.CursorPageResult[*model.Comment], error) {
 	// 构建基础查询 - 只查询顶级评论
 	query := r.db.WithContext(ctx).Model(&model.Comment{}).
 		Where("post_id = ? AND parent_id IS NULL AND status = ?", postID, model.CommentStatusPublished)
 
 	// 使用游标构建器
-	builder := cursor.NewBuilder(query, cursor.SortByCreatedAtDesc).
+	builder := cursor.NewBuilder(query, cursor.SortByCreatedAtAsc).
 		WithCursor(req.Cursor, req.Direction).
 		WithPageSize(req.PageSize)
 
@@ -539,7 +539,7 @@ func (r *commentRepository) GetCommentsByCursor(ctx context.Context, postID stri
 			nextCursor = cursor.NewCursor(
 				cursor.FormatTime(lastComment.CreatedAt),
 				lastComment.ID,
-				cursor.SortByCreatedAtDesc,
+				cursor.SortByCreatedAtAsc,
 			).Encode()
 		}
 		// 上一页游标
@@ -547,7 +547,7 @@ func (r *commentRepository) GetCommentsByCursor(ctx context.Context, postID stri
 		prevCursor = cursor.NewCursor(
 			cursor.FormatTime(firstComment.CreatedAt),
 			firstComment.ID,
-			cursor.SortByCreatedAtDesc,
+			cursor.SortByCreatedAtAsc,
 		).Encode()
 	}