CVE-2026-55219: Paymenter has race condition in payWithCredit() that enables credit double-spend

The credit payment implementation in app/Livewire/Invoices/Show.php executes a pessimistic row lock (lockForUpdate()) outside of an active database transaction. Because MySQL/MariaDB requires an enclosing transaction to enforce row-level locks, the guard is ineffective. Concurrent payment requests can exploit this race condition to read the same credit balance simultaneously, allowing users to pay multiple invoices using the same credit balance.