Sends an announcement (title + body) as a push and/or in-app notification, and records it in the "Messages" tab of the school's app. The endpoint validates and resolves external references, then dispatches asynchronously — a 200 confirms dispatch, not delivery.

Availability: this endpoint is only available to schools with a white-label app; it has no effect for schools without one.

Audience note: recipients.type "targeted_users" targets any user stored in the school's users table by their id or external reference — not only students. Some schools register non-student roles there so those users can access the white-label app, so the audience is not limited to actual students.