Fix `ready_to_import` query performance

💾 : Problem

The query generated by ContainerRepository.ready_for_import.take is using a Seq Scan: https://explain.depesz.com/s/9YXB

This is part of what has caused degradation in Phase 2 of the migration where we are not able to hit the capacity limits of the Enqueuer.

💿 Solution

We need to add/update indexing to improve the query plan We need to use a higher limit than one on the query to force the postgres planner to use the index.