Ledger Submit Describe

Self-describing, public, no-auth GET for the POST /ledger/submit door -- returns the exact request shape, price, and payment/registration flow so a third-party UI (e.g. a console rendering our submission door as a real, linkable step, not just prose) can point at something live instead of a static description. MUST be registered before GET /ledger/{entry} in this file -- FastAPI matches path routes in registration order, and a param route would otherwise swallow the literal string "submit" as an entry id (confirmed live: this exact 404 happened before this route existed, 2026-08-06, Merlini/trustless-ai console integration ask).