Generates a session-leader rotation order for every group in a peer_led cohort using IvsE and prep-preference scores extracted on demand from each participant's intake form_submissions.raw_payload.

The proposal is NOT persisted — the FE displays it and lets the CM review + edit per cell before committing the batch via POST /finalize-leader-rotation. Participants without a completed intake submission are surfaced under skipped_participants and excluded from the rotation rather than being assigned a neutral default that would bias the result.

Gated by the live_session_delivery feature flag.