Include Billing Information in Refunds Export

🔍 Problem Statement
As a finance admin, I experience the refunds export lacking billing information, which prevents direct reconciliation with original transactions and requires manual cross-referencing.

💡 User Story
As a finance admin, I want billing details (name, address, last 4 of payment method) included in the refunds export, so that I can reconcile refunds without pulling a second report.

🎯 Definition of Done
- Given the refunds export, when downloaded, then it includes billing name, billing address, and payment method (last 4 / type).
- This change impacts the API (refunds export endpoint).
- Limitations: fields are appended to the existing format; no existing columns are removed.