shopper/cart allows discount over‑redemption during checkout

## Impact

`CreateOrderFromCartAction::execute` previously created the `Order` row before checking and incrementing the discount's `total_use` counter. Under concurrent checkout pressure (Black Friday, flash sale, viral coupon), the global `usage_limit` was silently exceeded: orders were committed with the discount fully applied to `price_amount` while the counter blocked at `usage_limit`. The merchant had no signal that an over-redemption had occurred.

A second related bug: `usage_limit_per_user` was effectively a no-op because the counter it relied on (`DiscountDetail.total_use`) was never incremented anywhere in the codebase. The per-user check therefore always saw `0` uses and validation passed regardless of how many times the same customer had previously redeemed the coupon. For `eligibility = Everyone` the per-user limit could not fire at all because the underlying `DiscountDetail` row only exists for `eligibility = Customers`.

Direct financial loss: each over-redemption is a discount the merchant did not intend to grant.

## Patches

Fixed in `v2.8.0`. `CreateOrderFromCartAction` now:

- Reserves the discount slot atomically before the order row is created, inside the same `DB::transaction` with `lockForUpdate` and a compare-and-swap on `total_use`.
- Throws `DiscountLimitReachedException::global` and rolls back the transaction when the global limit was exhausted between cart validation and commit. No order is committed.
- Throws `DiscountLimitReachedException::perUser` and rolls back when the discount is restricted to one use per customer and the customer has already redeemed it.
- Snapshots `discount_id`, `discount_code`, `discount_type`, `discount_value_at_apply` and `discount_currency_code` onto the `orders` table for resilience against later discount edits or deletions.

`DiscountValidator` was updated to perform the same Order-based per-user check at cart-apply time so the rejection is surfaced before checkout.

Upgrade via:

`composer require shopper/cart:^2.8 shopper/core:^2.8`
`php artisan migrate`

## Workarounds

None. Upgrade to `v2.8.0`.

## Resources

- Issue: https://github.com/shopperlabs/shopper/issues/510
- Pull request: https://github.com/shopperlabs/shopper/pull/511
- CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization