Pricing Preview

Pre-checkout pricing quote for the human chat UI. Single source of truth for bundle discount + promotional discount math, so the pre-checkout quote the buyer sees always matches the actual /charge/create or /api/v1/stripe/checkout amount -- the frontend must never recompute bundle/discount math client-side (see estatetrace-design-spec.md 2026-10-05 note: a prior client-side-only implementation could show a buyer a stale total that didn't reflect an active promotional discount window).

lead_ids: comma-separated list of lead_id values (query param, not a POST body) since this is a lightweight idempotent GET fired on every checkbox change in the UI.