feat: user_post_views から id を削除（#230） (#239)

#230

Co-authored-by: miteruzo <miteruzo@naver.com>
Reviewed-on: #239

backend/app/models/user_post_view.rb

@@ -1,4 +1,6 @@
 class UserPostView < ApplicationRecord
+  self.primary_key = :user_id, :post_id
+
   belongs_to :user
   belongs_to :post