Stop resuming from the persisted (older) BoxAPI cursor. Resuming made every
sync walk further back in time and never re-fetch posts published since the
last run, so a seller's newest posts were never imported. Always start from
max_id=None (newest) and page older within a single run, de-duping against
the DB: brand-new posts first, then older un-imported ones.
Also emit timezone-aware UTC created_at so ordering is correct regardless of
the container's local timezone (and Django no longer receives naive datetimes).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
