ae7feeb726
On first startup with empty database, ensure_servers_synced() fetched
squads from RemnaWave but create_server_squad() failed with
ValueError('Server squad must be linked to at least one promo group')
because no default promo group existed yet.
Now _get_default_promo_group_id() auto-creates a default promo group
via _get_or_create_default_promo_group() when none exists, matching
the pattern used throughout the codebase for user registration.