Auto-cleanup Nouveau indexes

## Summary / Desired Behaviour / Possible Solution

Make Smoosh automatically [cleanup Nouveau indexes](https://docs.couchdb.org/en/latest/api/database/cleanup.html#post--db-_nouveau_cleanup) to optimize disk usage.

## Additional context

[Related Slack thread](https://couchdb.slack.com/archives/C49LEE7NW/p1732102415451979)

Start by tracing how Smoosh performs its existing cleanup work and read the linked Nouveau cleanup API documentation. Determine when automatic cleanup should run and verify that Smoosh invokes the Nouveau index cleanup endpoint without disrupting its other work.

Written by the indexing model from the issue text.