2nd email address field

🔍 Problem Statement

As a Pelcro admin, customers often have separate billing and access (login) email addresses, but the customer record exposes only one email field — so admins stuff the second address into Metadata, where it doesn't appear in exports or trigger the right communications.

💡 User Story

As an admin, I want a second, first-class email field on the customer record — a Billing email alongside the existing Main (access/login) email — so both addresses appear in exports and the right one receives the right notifications.

🎯 Definition of Done (DoD)

• ✔ Given a customer record, when an admin sets a Billing email, then invoices / payment-failure / receipt emails are sent to it (falling back to the Main email when unset).
• ✔ Both fields are returned by the Core API and included in customer/subscription exports.
• ✔ This change will impact UI / API — specifically the Customer profile (Platform UI), the Customer resource (Core API), and billing email templates.
• ✔ Limitations: exactly two email addresses supported; no shipping/notification email beyond Main + Billing in this scope.