Adds _generate_ai_description background task that fires after a
successful upload: images get a one-sentence vision description via
Claude, text/log/config files get extracted_content + AI summary when
>2000 chars. Runs as asyncio.create_task so it never blocks the upload
response. Errors are logged and swallowed.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>