fix transaction status logic inversion

Corrected inverted logic in createTransaction where payments were
marked as pending and requests as complete. Changed condition from
isRequestTransaction to isPayment to properly handle balance transfers
and status updates.

Fixes: Unit tests transactions.test.ts

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, {