feat: add daily cron job to mark expired invoices as overdue

[rahul-rajak-nsut]

What does this PR do?

Adds a daily cron job that automatically checks and updates
the expiry status of invoices every day at midnight.

Related Issues

Closes #417

Steps to Test

1. Start the backend server
2. Check console for message: "Invoice expiry cron job scheduled!"
3. Invoices with expiredDate < today will be marked as overdue automatically

Changes Made

• Created backend/src/cron/invoiceCron.js
• Installed node-cron package
• Registered cron job in backend/src/server.js

Checklist

• I have tested these changes
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings or errors
• The title of my pull request is clear and descriptive

[anktiwari053]

@rahul-rajak-nsut why we are not using agenda or node-schedule in place of node-cron.