fix inverted transaction status logic

Fixed critical bug where payment and request transaction logic was inverted.
Payments were incorrectly staying in pending status instead of completing,
while requests were being marked complete immediately on creation.

Changed createTransaction condition from isRequestTransaction() to isPayment()
to correctly process payments (debit/credit/complete) versus requests (pending).

All unit tests now passing.

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: kevintang513

backend/database.ts

@@ -546,7 +546,7 @@ export const createTransaction = (
   const savedTransaction = saveTransaction(transaction);
 
   // if payment, debit sender's balance for payment amount
-  if (isRequestTransaction(transaction)) {
+  if (isPayment(transaction)) {
     debitPayAppBalance(sender, transaction);
     creditPayAppBalance(receiver, transaction);
     updateTransactionById(transaction.id, {