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/test/java/com/openisle/controller/PostControllerTest.java

@@ -55,6 +55,10 @@ class PostControllerTest {
     private UserVisitService userVisitService;
     @MockBean
     private PostReadService postReadService;
+    @MockBean
+    private MedalService medalService;
+    @MockBean
+    private com.openisle.repository.PollVoteRepository pollVoteRepository;
 
     @Test
     void createAndGetPost() throws Exception {
@@ -63,9 +67,13 @@ class PostControllerTest {
         Category cat = new Category();
         cat.setId(1L);
         cat.setName("tech");
+        cat.setDescription("Technology category");
+        cat.setIcon("tech-icon");
         Tag tag = new Tag();
         tag.setId(1L);
         tag.setName("java");
+        tag.setDescription("Java programming language");
+        tag.setIcon("java-icon");
         Post post = new Post();
         post.setId(1L);
         post.setTitle("t");
@@ -111,9 +119,13 @@ class PostControllerTest {
         Category cat = new Category();
         cat.setId(1L);
         cat.setName("tech");
+        cat.setDescription("Technology category");
+        cat.setIcon("tech-icon");
         Tag tag = new Tag();
         tag.setId(1L);
         tag.setName("java");
+        tag.setDescription("Java programming language");
+        tag.setIcon("java-icon");
         Post post = new Post();
         post.setId(1L);
         post.setTitle("t2");
@@ -147,9 +159,13 @@ class PostControllerTest {
         Category cat = new Category();
         cat.setId(1L);
         cat.setName("tech");
+        cat.setDescription("Technology category");
+        cat.setIcon("tech-icon");
         Tag tag = new Tag();
         tag.setId(1L);
         tag.setName("java");
+        tag.setDescription("Java programming language");
+        tag.setIcon("java-icon");
         Post post = new Post();
         post.setId(2L);
         post.setTitle("hello");
@@ -197,9 +213,13 @@ class PostControllerTest {
         Category cat = new Category();
         cat.setId(1L);
         cat.setName("tech");
+        cat.setDescription("Technology category");
+        cat.setIcon("tech-icon");
         Tag tag = new Tag();
         tag.setId(1L);
         tag.setName("java");
+        tag.setDescription("Java programming language");
+        tag.setIcon("java-icon");
         Post post = new Post();
         post.setId(1L);
         post.setTitle("t");
@@ -262,6 +282,8 @@ class PostControllerTest {
         Category cat = new Category();
         cat.setId(1L);
         cat.setName("tech");
+        cat.setDescription("Technology category");
+        cat.setIcon("tech-icon");
         Post post = new Post();
         post.setId(1L);
         post.setTitle("t");

backend/src/test/java/com/openisle/service/PostCommentStatsTest.java

@@ -0,0 +1,90 @@
+package com.openisle.service;
+
+import com.openisle.model.*;
+import com.openisle.repository.*;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.test.context.TestPropertySource;
+import org.springframework.transaction.annotation.Transactional;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+@SpringBootTest
+@TestPropertySource(locations = "classpath:application.properties")
+@Transactional
+public class PostCommentStatsTest {
+
+    @Autowired
+    private PostRepository postRepository;
+
+    @Autowired
+    private UserRepository userRepository;
+
+    @Autowired
+    private CategoryRepository categoryRepository;
+
+    @Autowired
+    private TagRepository tagRepository;
+
+    @Autowired
+    private CommentService commentService;
+
+    @Test
+    public void testPostCommentStatsUpdate() {
+        // Create test user
+        User user = new User();
+        user.setUsername("testuser");
+        user.setEmail("test@example.com");
+        user.setPassword("hash");
+        user = userRepository.save(user);
+
+        // Create test category
+        Category category = new Category();
+        category.setName("Test Category");
+        category.setDescription("Test Category Description");
+        category.setIcon("test-icon");
+        category = categoryRepository.save(category);
+
+        // Create test tag
+        Tag tag = new Tag();
+        tag.setName("Test Tag");
+        tag.setDescription("Test Tag Description");
+        tag.setIcon("test-tag-icon");
+        tag = tagRepository.save(tag);
+
+        // Create test post
+        Post post = new Post();
+        post.setTitle("Test Post");
+        post.setContent("Test content");
+        post.setAuthor(user);
+        post.setCategory(category);
+        post.getTags().add(tag);
+        post.setStatus(PostStatus.PUBLISHED);
+        post.setCommentCount(0L);
+        post = postRepository.save(post);
+
+        // Verify initial state
+        assertEquals(0L, post.getCommentCount());
+        assertNull(post.getLastReplyAt());
+
+        // Add a comment
+        commentService.addComment("testuser", post.getId(), "Test comment");
+
+        // Refresh post from database
+        post = postRepository.findById(post.getId()).orElseThrow();
+
+        // Verify comment count and last reply time are updated
+        assertEquals(1L, post.getCommentCount());
+        assertNotNull(post.getLastReplyAt());
+
+        // Add another comment
+        commentService.addComment("testuser", post.getId(), "Another comment");
+
+        // Refresh post again
+        post = postRepository.findById(post.getId()).orElseThrow();
+
+        // Verify comment count is updated
+        assertEquals(2L, post.getCommentCount());
+    }
+}