Billing contacts: add, delete and invoice CC

Finance teams asked for a way to route invoices to accounts payable without giving every accountant a dashboard seat, so we’ve added a separate list of billing contacts for each organization.

• Add or remove billing contacts at the organization level without affecting member or admin access.
• Billing contacts are automatically CC’d on invoice emails when invoices are generated.
• The primary billing email is excluded from the contact list so the same person doesn’t receive duplicate copies.