Sends a recap email to the listed session participants. The coach composes Subject + Body HTML in the "Send Recap Email" modal and may include any of the seven recap-email merge tags ({{first_name}}, {{cohort_name}}, {{session_number}}, {{session_date}}, {{group_name}}, {{topic_name}}, {{coach_name}}). Tags are resolved server-side per recipient — null values become empty strings, so the literal tag text never reaches a participant (AC15).

Recipients are intersected with the session's group membership before sending — IDs not in the group are silently dropped and reported in skipped_not_in_session.

Authorization: only the coach assigned to the session's group can call this endpoint. Calls from other coaches receive 403. 1:1 sessions (no group) also receive 403 — recap email is group-only this story.

MED-893 (post-session AI features) flows the AI-generated body through this same endpoint without changes.