Core API: Refund support

🔍 Problem Statement

The Pelcro Core API does not currently support issuing refunds programmatically. Clients who need to process refunds must do so manually through the dashboard or directly via their payment gateway, which slows resolution and prevents automation of refund workflows.

💡 User Story

As a developer integrating with Pelcro, I want a Core API endpoint to issue refunds, so that I can automate refund processing without manual dashboard intervention or direct gateway calls.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given a paid invoice or payment, when a developer calls the refund API endpoint with valid parameters, then the refund is processed through the payment gateway, the invoice/payment status is updated in Pelcro, and the transaction is reflected in accounting reports.