I resolved this by deleting the existing postgres folders (actually I moved them to temporary storage and still need to delete them), rebuilding, and then restoring the recent backup.
Even after I deleted the database, I still got an error with the AI plugin, so I removed it and then was able to rebuild the empty site and restore the backup. I did a rebuild after that, just to be sure.
My hypothesis is that somehow the database and the migration record got out of sync.
This site was migrated from CDCK around June.