fix: 旧帖子的last_reply_at也要及时更新（仅一次）

backend/src/main/java/com/openisle/mapper/PostMapper.java

@@ -83,7 +83,10 @@ public class PostMapper {
         dto.setParticipants(participants.stream().map(userMapper::toAuthorDto).collect(Collectors.toList()));
 
         LocalDateTime last = post.getLastReplyAt();
-        dto.setLastReplyAt(last != null ? last : post.getCreatedAt());
+        if (last == null) {
+            commentService.updatePostCommentStats(post);
+        }
+        dto.setLastReplyAt(post.getLastReplyAt());
         dto.setReward(0);
         dto.setSubscribed(false);
         dto.setType(post.getType());

backend/src/main/java/com/openisle/service/CommentService.java

@@ -346,12 +346,16 @@ public class CommentService {
     /**
      * Update post comment statistics (comment count and last reply time)
      */
-    private void updatePostCommentStats(Post post) {
+    public void updatePostCommentStats(Post post) {
         long commentCount = commentRepository.countByPostId(post.getId());
-        LocalDateTime lastReplyAt = commentRepository.findLastCommentTime(post);
-        
         post.setCommentCount(commentCount);
-        post.setLastReplyAt(lastReplyAt);
+
+        LocalDateTime lastReplyAt = commentRepository.findLastCommentTime(post);
+        if (lastReplyAt == null) {
+            post.setLastReplyAt(post.getCreatedAt());
+        } else {
+            post.setLastReplyAt(lastReplyAt);
+        }
         postRepository.save(post);
         
         log.debug("Updated post {} stats: commentCount={}, lastReplyAt={}",