fix: server squad sync fails on fresh DB without default promo group

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.

app/database/crud/server_squad.py

@@ -33,7 +33,15 @@ logger = structlog.get_logger(__name__)
 
 async def _get_default_promo_group_id(db: AsyncSession) -> int | None:
     result = await db.execute(select(PromoGroup.id).where(PromoGroup.is_default.is_(True)).limit(1))
-    return result.scalar_one_or_none()
+    default_id = result.scalar_one_or_none()
+    if default_id is not None:
+        return default_id
+
+    # На пустой БД дефолтной промогруппы нет — создаём автоматически
+    from app.database.crud.user import _get_or_create_default_promo_group
+
+    default_group = await _get_or_create_default_promo_group(db)
+    return default_group.id
 
 
 async def create_server_squad(