When changing an order status to « Annulé », check if the related
invoice has any validated payments via ``Invoices::getPayments()``.
If the total paid is greater than zero, the cancellation is refused
with a growl error showing the amount already paid.
Add ``restoreStock()``, ``decreaseStockForOrder()``, and
``checkStockForOrder()`` to ``OfferStocks``. When an order is cancelled
(status 4), stock is restored. When un-cancelling, stock availability is
checked first—insufficient stock blocks the transition with an error
message—then decremented.
