No stock validation existed in the ordering flow, allowing customers
to order more items than available.
Cap quantity to ``stock_current`` in ``Baskets::getBasketData()`` when
adding to cart. Add ``min=1`` and ``max=stock`` attributes on the
basket quantity input, with JS clamping in the change handler.
Verify stock again in ``Shop\OrderController::store()`` before saving
the order as a race-condition safeguard.
