Add category module and require post category

src/main/java/com/openisle/service/PostService.java

@@ -2,8 +2,10 @@ package com.openisle.service;
 
 import com.openisle.model.Post;
 import com.openisle.model.User;
+import com.openisle.model.Category;
 import com.openisle.repository.PostRepository;
 import com.openisle.repository.UserRepository;
+import com.openisle.repository.CategoryRepository;
 import lombok.RequiredArgsConstructor;
 import org.springframework.stereotype.Service;
 
@@ -14,14 +16,18 @@ import java.util.List;
 public class PostService {
     private final PostRepository postRepository;
     private final UserRepository userRepository;
+    private final CategoryRepository categoryRepository;
 
-    public Post createPost(String username, String title, String content) {
+    public Post createPost(String username, Long categoryId, String title, String content) {
         User author = userRepository.findByUsername(username)
                 .orElseThrow(() -> new IllegalArgumentException("User not found"));
+        Category category = categoryRepository.findById(categoryId)
+                .orElseThrow(() -> new IllegalArgumentException("Category not found"));
         Post post = new Post();
         post.setTitle(title);
         post.setContent(content);
         post.setAuthor(author);
+        post.setCategory(category);
         return postRepository.save(post);
     }