'backend/config/storage.yml' を更新

--- a/backend/config/storage.yml
+++ b/backend/config/storage.yml
@@ -13,3 +13,4 @@ r2:
  secret_access_key: <%= ENV['R2_SECRET_ACCESS_KEY'] %>
  bucket: <%= ENV['R2_BUCKET'] %>
  region: auto
  request_checksum_calculation: when_required