feat: add draft post support

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

@@ -0,0 +1,58 @@
+package com.openisle.service;
+
+import com.openisle.model.Category;
+import com.openisle.model.Draft;
+import com.openisle.model.Tag;
+import com.openisle.model.User;
+import com.openisle.repository.CategoryRepository;
+import com.openisle.repository.DraftRepository;
+import com.openisle.repository.TagRepository;
+import com.openisle.repository.UserRepository;
+import lombok.RequiredArgsConstructor;
+import org.springframework.stereotype.Service;
+
+import java.util.HashSet;
+import java.util.List;
+import java.util.Optional;
+import java.util.Set;
+
+@Service
+@RequiredArgsConstructor
+public class DraftService {
+    private final DraftRepository draftRepository;
+    private final UserRepository userRepository;
+    private final CategoryRepository categoryRepository;
+    private final TagRepository tagRepository;
+
+    public Draft saveDraft(String username, Long categoryId, String title, String content, List<Long> tagIds) {
+        User user = userRepository.findByUsername(username)
+                .orElseThrow(() -> new IllegalArgumentException("User not found"));
+        Draft draft = draftRepository.findByAuthor(user).orElse(new Draft());
+        draft.setAuthor(user);
+        draft.setTitle(title);
+        draft.setContent(content);
+        if (categoryId != null) {
+            Category category = categoryRepository.findById(categoryId)
+                    .orElseThrow(() -> new IllegalArgumentException("Category not found"));
+            draft.setCategory(category);
+        } else {
+            draft.setCategory(null);
+        }
+        Set<Tag> tags = new HashSet<>();
+        if (tagIds != null && !tagIds.isEmpty()) {
+            tags.addAll(tagRepository.findAllById(tagIds));
+        }
+        draft.setTags(tags);
+        return draftRepository.save(draft);
+    }
+
+    public Optional<Draft> getDraft(String username) {
+        return userRepository.findByUsername(username)
+                .flatMap(draftRepository::findByAuthor);
+    }
+
+    public void deleteDraft(String username) {
+        userRepository.findByUsername(username)
+                .ifPresent(draftRepository::deleteByAuthor);
+    }
+}