User Story: Make balance check atomic in src/javascript/backend/api/payment.js

[aliAljaffer]

As a developer
I want to fix the balance check and update flow in src/javascript/backend/api/payment.js to avoid race conditions
So that concurrent payment requests cannot cause incorrect "Insufficient balance" errors or double-charging

Acceptance Criteria

• Ensure balance retrieval and deduction are performed atomically (using DB transactions or row-level locking) instead of separate read-then-write steps.

Details
The code checks balance then later deducts it; comments indicate a race condition and the code throws Insufficient balance in a non-atomic flow.