Backend: rate limiter gains check_with_retry() returning seconds until
the next slot opens. Upload 429 responses include retry_after_secs in
JSON and a Retry-After header.
Frontend: upload queue catches 429 as RateLimitError, resets affected
item to pending, schedules processQueue() for the server-reported delay,
and shows a live countdown banner in the queue UI.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
de0e395a9e