Add a server

Queues a single server for discovery. Auth is optional: send Discovery credentials to skip the rate limit, or call anonymously and take the limit. We dial the server before answering, so the status code tells you what actually happened: 201 if it is reachable and freshly added, 208 if we already knew it, 422 if it would not answer (we record it offline rather than re-dial it on every retry).