feat(notification): add default email newsletter enrollment policy for new users

[JoseSzycho]

Summary

• Adds a new notification.miloapis.com service configuration under config/services/.
• Introduces a ContactGroupEnrollmentPolicy (email-newsletter-enrollment-policy) that automatically enrolls all new User-backed Contacts into the email newsletter ContactGroup on ContactCreated.
• Wires the new service into the top-level config/services/kustomization.yaml.

Details

The policy targets all contacts with subjectKind: User, so every new user signup triggers enrollment into the configured ContactGroup without any manual intervention. The ContactGroup ref (emailnewsletter-contact-group-3a3ar9 in default) can be updated as needed.

This depends on the ContactGroupEnrollmentPolicy CRD and controller shipped in milo#603.

Test plan

• Verify kustomize build config/services renders cleanly with the new component.
• Deploy to a test environment and confirm new User creation results in a ContactGroupMembership for the newsletter group.

🤖 Generated with Claude Code

Related to:

• feat: Introduce ContactGroupEnrollmentPolicy and controller to automate contact group memberships. milo-os/milo#603 (review)