Backend: extend rate limiter with check_with_retry() that returns the
seconds until the next slot opens. Upload 429 responses now include
retry_after_secs in the JSON body and a Retry-After header.
Frontend: the upload queue catches 429s as RateLimitError, resets the
affected item to pending, schedules processQueue() to resume after the
server-reported wait, and shows a live countdown banner in UploadQueue.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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