feat: allow tariff deletion with active subscriptions

Remove blocking check that prevented tariff deletion when subscriptions
exist. DB schema already supports SET NULL on tariff FK, so subscriptions
gracefully become "legacy" and users pick a new tariff on renewal.
Return affected_subscriptions count in API response.

app/cabinet/routes/admin_tariffs.py

@@ -412,21 +412,14 @@ async def delete_existing_tariff(
             detail='Tariff not found',
         )
 
-    # Check if tariff has subscriptions
     subs_count = await get_tariff_subscriptions_count(db, tariff_id)
-    if subs_count > 0:
-        raise HTTPException(
-            status_code=status.HTTP_400_BAD_REQUEST,
-            detail=f'Cannot delete tariff with {subs_count} active subscriptions',
-        )
-
     await delete_tariff(db, tariff)
-    logger.info(f'Admin {admin.id} deleted tariff {tariff_id}: {tariff.name}')
+    logger.info(f'Admin {admin.id} deleted tariff {tariff_id}: {tariff.name} (affected subscriptions: {subs_count})')
 
     # Перезагружаем периоды из БД для синхронизации с ботом
     await load_period_prices_from_db(db)
 
-    return {'message': 'Tariff deleted successfully'}
+    return {'message': 'Tariff deleted successfully', 'affected_subscriptions': subs_count}
 
 
 @router.post('/{tariff_id}/toggle', response_model=TariffToggleResponse)