feat(posts): 优化帖子评论统计性能

- 在 Post 模型中添加 commentCount 和 lastReplyAt 字段
- 在 CommentService 中实现更新帖子评论统计的方法
- 在 PostMapper 中使用 Post 模型中的评论统计字段
- 新增数据库迁移脚本，添加评论统计字段和索引
- 更新相关测试用例

backend/src/main/resources/db/migration/V5__add_comment_stats_to_posts.sql

@@ -0,0 +1,19 @@
+-- Add comment count and last reply time fields to posts table for performance optimization
+ALTER TABLE posts ADD COLUMN comment_count BIGINT NOT NULL DEFAULT 0;
+ALTER TABLE posts ADD COLUMN last_reply_at DATETIME(6) NULL;
+
+-- Add index on last_reply_at for sorting by latest reply
+CREATE INDEX idx_posts_last_reply_at ON posts(last_reply_at);
+
+-- Initialize comment_count and last_reply_at with existing data
+UPDATE posts p SET 
+    comment_count = (
+        SELECT COUNT(*) 
+        FROM comments c 
+        WHERE c.post_id = p.id AND c.deleted_at IS NULL
+    ),
+    last_reply_at = (
+        SELECT MAX(c.created_at) 
+        FROM comments c 
+        WHERE c.post_id = p.id AND c.deleted_at IS NULL
+    );