19/19: pricing in the public serializer, checkout gated on agreements, server-authoritative
amount, webhook signature verification and idempotency, payment success/failure/abandonment,
last-seat checkout race, reservation reaper, manual refund with no cascade, minor guardian
routing, admin/RBAC payment-data restrictions, audit/metric hygiene, the status-lookup endpoint,
and confirmation that ROOT_ V1/free-offering/classroom flows are unaffected.
