feat: URL 正規化（#208） (#226)

#208

Co-authored-by: miteruzo <miteruzo@naver.com>
Reviewed-on: #226

backend/db/migrate/20260118144400_add_unique_index_to_url_in_posts.rb

@@ -0,0 +1,6 @@
+class AddUniqueIndexToUrlInPosts < ActiveRecord::Migration[7.1]
+  def change
+    change_column :posts, :url, :string, limit: 768
+    add_index :posts, :url, unique: true, name: 'index_posts_on_url'
+  end
+end