Fix underpayment alerts on unpaid invoices

[charlie12520]

Summary

• Short-circuit underpayment detection when an invoice has no active payment activity yet.
• Keep outstanding-balance summary math unchanged so fresh invoices still show the unpaid amount.
• Add coverage for fresh unpaid invoices and real partial-payment underpayments.

Fixes #72

Tests

• php artisan test tests/Unit/InvoicePaymentSummaryTest.php (5 passed, 28 assertions)

Local note: I ran the test inside the Sail Docker Compose service. The repo files were not changed for environment setup.