Embargoed dandisets lack aggregated asset stats

As reported on May 14 via `help@dandiarchive.org`, unembargoing a Dandiset resulted in an `Assets Summary` section with `This Dandiset does not contain any valid assets.` Satra was able to resolve the issue for the Dandiset reported by updating the Dandiset metadata, which triggered validation and generated a correct asset summary. Perhaps there broader issue that needs to be resolved for the unembargo process.

Start by reproducing the unembargo flow for a Dandiset and trace how it differs from the metadata update that triggered validation and generated the correct Assets Summary. Compare the two paths and identify where aggregated asset statistics are skipped. Done means unembargoing produces a correct asset summary without requiring a separate metadata update.

Written by the indexing model from the issue text.