MarkItDown.convert() does blocking file I/O and CPU-intensive PDF parsing.
This was blocking the event loop during file uploads.
Now wraps the entire markitdown pipeline (tempfile write, convert, cleanup)
in asyncio.to_thread() to run in thread pool.
🐾 Generated with [Letta Code](https://letta.com)
Co-authored-by: Letta <noreply@letta.com>
