Billing Email Inclusion on Past Due Invoice Reporting

🔍 Problem Statement
As a collections admin, I experience past-due invoice exports missing the billing email address, which forces time-consuming manual cross-referencing with other reports to identify who to contact.

💡 User Story
As an admin, I want the billing email included in the past-due invoice export, so that I can reach the right contact immediately without a secondary data pull.

🎯 Definition of Done
- Given the past-due invoices export, when downloaded, then a billingEmail column is included for each row.
- This change impacts the API (invoice export endpoint).
- Limitations: billing email may differ from the account email; both can be included if applicable.

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Presently, when we run an export of the past-due invoices, we cannot get the billing email on the same report, which is who we need to contact regarding the past-due invoices. While we can cross-reference other exports and ID numbers, this is extremely time consuming and does not need to be, when the addition of the Billing Email could be added to the invoice exports.