Completes the phase-1 backend client: password-grant OAuth with refresh-token
renewal (falling back to a full re-grant), anonymous public browsing when no
creds are set, list_channels, paginated channel_videos, and on-demand
video_media (direct MP4, None for HLS-only). Wires the tested pure mapping fns
to the API. The client is dead-code-allowed until the editor (phase 2) and
browse UI (phase 3) construct it.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
