POST /sessions → session_id
POST /sessions/:id/trigger → set trigger
POST /sessions/:id/nodes → add nodes
POST /sessions/:id/edges → connect nodes
POST /sessions/:id/forms → create form drafts
PATCH /sessions/:id/nodes/:node_id → wire form_id into nodes
POST /sessions/:id/portals → create portal drafts
POST /sessions/:id/validate → validate. Fix errors. Repeat.
POST /sessions/:id/activate → commit to production