Add queue-based background download system to improve performance and reliability:
- Introduce DOWNLOAD_SEMAPHORE to limit concurrent downloads to 3
- Add asyncio.Queue (maxsize 100) with dedicated worker for background processing
- Implement safe_get_messages and safe_download_media wrappers with timeout protection (30s and 120s)
- Add retry logic for KeyError auth failures with 5s backoff
- Replace synchronous sequential downloads with asynchronous queued processing
- Prevent event loop blocking by queuing files instead of immediate download
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