Add category module and require post category

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

@@ -0,0 +1,33 @@
+package com.openisle.service;
+
+import com.openisle.model.Category;
+import com.openisle.repository.CategoryRepository;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+
+import java.util.List;
+
+@Service
+@RequiredArgsConstructor
+public class CategoryService {
+    private final CategoryRepository categoryRepository;
+
+    public Category createCategory(String name) {
+        Category category = new Category();
+        category.setName(name);
+        return categoryRepository.save(category);
+    }
+
+    public void deleteCategory(Long id) {
+        categoryRepository.deleteById(id);
+    }
+
+    public Category getCategory(Long id) {
+        return categoryRepository.findById(id)
+                .orElseThrow(() -> new IllegalArgumentException("Category not found"));
+    }
+
+    public List<Category> listCategories() {
+        return categoryRepository.findAll();
+    }
+}

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);
     }