Archive a course matched by the external reference id (API_ID) set by your ERP. Same effects, errors and webhooks as DELETE /courses/:id.

The reference is not unique in legacy data, so the rule is: no course carries it → 404 COURSE_NOT_FOUND; only archived courses carry it → 409 COURSE_ALREADY_ARCHIVED; exactly one ACTIVE course carries it → that course is archived (archived courses sharing the reference never block); two or more ACTIVE courses carry it → 409 COURSE_EXTERNAL_REFERENCE_NOT_UNIQUE, nothing is archived and no webhook is sent: errors[] lists every active course (context.id, most recently updated first), so the caller archives the right one with DELETE /courses/:id or gives each course its own reference with PATCH /courses/:id.