Create cohort participant record and send welcome email

Signed MOUs often don't create roster entries, leaving coordinators without confirmed participants. This adds roster records and sends welcome emails so coordinators can start onboarding the same day.

Overview

When an MOU is marked signed, this flow turns a status change into a confirmed roster entry and an immediate welcome email. That removes manual copying and lets program coordinators begin onboarding participants the same day, keeping cohort launches on schedule.