The backend now returns an absolute pre-signed OSS URL for checkpoint
archives, and the publish/unpublish endpoints are live. Replace the
httpx + X-API-Key + base-url-concat download snippets with a plain
urllib.request.urlretrieve(archive.url), and drop the "not yet supported"
notices from the publish docs and REST/Service client references.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>