CVE-2026-86744: fallo de gravedad baja en grokability snipe-it

Snipe-IT 8.6.3 and earlier (and develop pre-release commits prior to the fix) contain a race condition in the asset checkout paths. Api\AssetsController::checkout() and Assets\AssetCheckoutController::store() call Asset::availableForCheckout() outside the mutation path and then invoke Asset::checkOut() without taking a row lock or re-checking availability, so two concurrent checkout requests for the same available asset can both observe it as available and both commit. This produces duplicate checkout-history rows, a doubled checkout_counter, and two CheckoutableCheckedOut events for a single-assignment asset, corrupting the audit trail and utilization/reconciliation reporting; the asset's final assigned_to remains singular, so the visible assignment stays intact. Exploitation requires an authenticated session holding the assets.checkout permission (or superuser) and precise concurrent timing. Fixed in 8.7.0.