Bulk-writes session leader assignments from a (possibly CM-edited) rotation proposal. Each assignment is validated against the MED-812 chain (cohort/session linkage, peer_led gate, lock cutoff, group membership) and written inside a single transaction so the finalize click is atomic. Each per-session write also records a session_audit_log row.

Once finalized, subsequent edits go through the existing per-cell endpoint PUT /api/cohorts/:cohortId/sessions/:sessionId/leader.

Gated by the live_session_delivery feature flag.