Lists comments for a specific user, ordered newest first by creation time with content_uuid descending as a tie-break. Pinning is per-container and is not applied to this cross-section feed. Pass the content_uuid of the last item received as starting_from to fetch the next, older page; omit it for the first page. A starting_from that does not resolve to content by this user -- unknown, belonging to another user, or deleted between pages -- returns an empty final page with more_available false, ending the feed rather than restarting it. more_available indicates whether further older items exist.