- Wrap get_content_from_url() in asyncio.to_thread() inside
get_sources_from_items() to prevent sync requests.get() from blocking
the event loop when users attach URL sources to chat messages.
The same function was already properly wrapped at retrieval.py:1839.
- Wrap hashlib.sha256(contents).hexdigest() in asyncio.to_thread()
inside upload_file_handler() to prevent CPU-bound hashing from
blocking the event loop during file uploads (44ms/100MB, scales
linearly with file size).