Requeue BBM to backfill namespace_details

As discussed here #358144 (comment 2803627423), we need to backfill the namespace_details table with the description, description_html and cached_markdown_version from the namespaces table where the namespace_details records are missing.

Also refer to #358144 (comment 2807780667) on how to avoid queueing it if it's already enqueued.