ICYLAB Billing Payment Write Safety v6.15

Files:
- services/billing/BillingReceivablesService.php
- services/billing/BillingPayablesService.php
- modules/billing/receivable_register_payment_ajax.php
- modules/billing/payable_register_payment_ajax.php
- modules/billing/receivables.js
- modules/billing/payables.php
- modules/billing/customer_aging.js
- modules/billing/suppliers_aging.js
- scripts/billing_payment_write_safety_diagnostic_v6_15.php
- database/migrations/2026_07_14_billing_payment_idempotency_v6_15.sql

Production bulk payments remain blocked.
