feat(inventory): add validation for item availability in released reservations

- Implement `check_released_reservations_available` function to verify if items from released reservations are still available for re-sale when attempting to change a returned order's status
- Update `create_sale_on_order_completion` signal to use this check, allowing transitions to positive statuses only if items are available, otherwise blocking with ValidationError
- Wrap Order.save() in transaction.atomic() to ensure ValidationError in signals rolls back the save operation
- Add comprehensive tests for scenarios where items are available or used in other orders
- Update date carousel in order to always center on today's date and remove unnecessary saving logic
- Add test flag to Django Debug Toolbar settings

Closes #123 (assuming related issue)

myproject/orders/models/order.py

@@ -176,6 +176,7 @@ class Order(models.Model):
         return reverse('orders:order-detail', kwargs={'order_number': self.order_number})
 
     def save(self, *args, **kwargs):
+        from django.db import transaction
         # Генерируем уникальный номер заказа при создании (начиная с 100 для 3-значного поиска)
         if not self.order_number:
             last_order = Order.objects.order_by('-order_number').first()
@@ -184,7 +185,10 @@ class Order(models.Model):
                 self.order_number = max(last_order.order_number + 1, 100)
             else:
                 self.order_number = 100
-        super().save(*args, **kwargs)
+
+        # Оборачиваем в транзакцию чтобы ValidationError в сигналах откатывал save()
+        with transaction.atomic():
+            super().save(*args, **kwargs)
 
     def recalculate_amount_paid(self):
         """