rake task to import last DB backup to development DB.

Warning: this *erases* all data in that DB.
Change config.backup_dir to a Pathname so #join can be used on it.
Use the new Backup model for backups from the CLI to avoid duplication.
Add a transient @auto attribute to Backup.

That way „automatic“ backups (created before data import) and those
triggered via rake can be distinguished.