POST /payments/quick had no dedupe and non-atomic INSERT+applyBalanceDelta,
unlike /payments/bulk. A double-click/retry/two-tab pay created a second
payment and dropped the balance twice; a mid-way failure left a payment with
no balance adjustment. Now checks the bill_id+paid_date+amount composite key
(idempotent 200) and wraps the write in db.transaction.
Test: tests/paymentsQuickRoute.test.js
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
null