Backend generates thumbnails on-demand via Celery, so the first request
often 404s while the worker runs. Auto-retry with 1.5s/3.5s/6s backoff,
manual retry fallback, and proper timer cleanup so fast-scrolling a
virtualized timeline doesn't setState on unmounted components.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
