Plan Build Stream

SSE relay of an onboarding plan build's data-plan-* deltas, so the trip page fills in as the editor writes it.

Same Redis-Streams → SSE plumbing as the chat stream and the social import, and deliberately NOT a chat stream: there is no conversation behind this and no resume cursor to keep. Plan deltas are idempotent by id, so a reader that attaches late simply replays from the top.

Registered above /{plan_id}/… — "build" would otherwise be captured as a plan id by the route matcher.