fix(relay): device login should back off on 429/slow_down instead of exiting #174

Summary

device login exits instead of backing off while polling a device authorization. During the RC4 campaign setup (#164), four device logins were started concurrently for four state directories. All four exited while authorization_pending with control_request_refused and httpStatus: 429, which invalidated their user codes. One earlier attempt also exited on a transient control_request timeout during polling.

Expected (RFC 8628 polling)

• On slow_down or HTTP 429, increase the polling interval (honoring Retry-After when present) and keep polling until the device code expires.
• Retry transient network timeouts during polling within the code lifetime. Polling is idempotent.
• Exit only on terminal errors (access_denied, expired_token, invalid client, …).

Acceptance

• Tests for 429/slow_down backoff, transient timeout retry and terminal errors.
• Output never includes tokens, and the user code is shown only as today.