fix(db): make blog cleanup migration idempotent

db/migrations/V021__FixBlogPostsAdvertisingCompliance.sql

@@ -197,7 +197,8 @@ $$,
   1,
   true,
   NOW()
-);
+)
+ON CONFLICT (slug) DO NOTHING;
 
 -- 2. 이번달 부가가치세 신고
 INSERT INTO blog_posts (title, slug, content, category_id, is_published, created_at)
@@ -399,7 +400,8 @@ $$,
   1,
   true,
   NOW()
-);
+)
+ON CONFLICT (slug) DO NOTHING;
 
 -- 3. 프리랜서를 위한 종합소득세 신고
 INSERT INTO blog_posts (title, slug, content, category_id, is_published, created_at)
@@ -636,3 +638,4 @@ $$,
   true,
   NOW()
 );
+