Brevo newsletter (un)subscribe (via SDK)

Problem/Motivation

Provide an API to (un)subscribe from newsletters programmatically as the foundation for UI features.

1. Provide Brevo SDK integration https://github.com/getbrevo/brevo-php
2. Provide UI to enter the API Key
3. Allow other modules to interact with the authenticated SDK

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes