MED-1196 (B3) — manual retention purge. Deletes every persisted AI-OUTPUT store for the cohort, across all of its sessions: session transcripts, airtime aggregates, discussion themes, and recap email drafts. Issues real deletes per store (does not rely on the sessions FK cascade, which a soft-delete does not trip). Does NOT touch core cohort, session, participant-entered, or configuration data — including the kept pre-session synthesis, prep AI-Note suggestions, and prep AI-prompt selections.

Authorization is internal_admin persona only, enforced server-side. Coaches, cohort managers (internal_user), and participants cannot invoke it. The endpoint is gated behind the live_session_delivery feature flag and returns 404 when the flag is off. Idempotent: a cohort with no persisted AI data returns success with zero counts.