2x Improvement requests for the storage migration endpoint

[YasenT]

I have been using the storage migrate endpoint to switch from OCI storage to S3.
While it worked okay for small domains, when I hit a domain with more artifacts (160K), some flaws popped up.

• The migration running artifact by artifact is slow for big domain. Some concurrency might be helpful.
• The migration is currently "locking" the whole domain for the duration of it. Which for bigger domains becomes service affecting, not being able to perform anything for 24 hours for example.

I created this draft PR. Feel free to close it if you find the approach bad, it's just something that Claude suggested. #7807