Include "invitation email" in Core API for memberships. We should be able to filter by subscription id and/or invitation email to find memberships in "pending" status. The invitation email address should be also be included as part of the response.

🔍 Problem Statement

The Core API for memberships does not include the invitation email field. Developers cannot programmatically retrieve or set the invitation email, which is needed for membership provisioning workflows.

💡 User Story

As a developer, I want the invitation email included in the Core API membership response and settable via the API, so that I can build automated membership provisioning without manual dashboard steps.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given a membership API request, when the response is returned, then it includes the invitation email field — and when creating or updating a membership, the invitation email can be set via the API.