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.

Problem Statement Currently, the refunds export does not include billing information, which makes it difficult to reconcile refunds with original transactions and customer records. This creates gaps in financial tracking and increases manual effort for teams trying to match refunds with their corresponding billing details. User Story As a finance or operations user, I want billing information to appear in the refunds export, so that I can accurately reconcile refunds with the original transactions and maintain consistent financial records without manual lookup.