HTTP/JSON server hosted on top of Axum. Exposes the request/response and subscribe-via-streaming endpoints in
a transport that is friendlier than gRPC for browsers, scripting languages, and tooling that does not want to
depend on protobuf. Routing, handlers, and per-route state sit on top of sub-server's shared dispatch.
JSON payloads come from wire-format's JSON encoder; the HTTP path is the canonical place RBCF gets rendered
into something a generic client can consume. Anything that needs raw RBCF should use the gRPC or WebSocket
transports instead.