perf(retrieval): make URL fetch and file hash non-blocking with asyncio.to_thread (#25822)

- 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).

backend/open_webui/routers/files.py

@@ -341,7 +341,9 @@ async def upload_file_handler(
 
         # SHA-256 of raw uploaded bytes for incremental sync diffing.
         # If the client pre-computed and sent file_hash, use that.
-        file_hash = file_metadata.get('file_hash') or hashlib.sha256(contents).hexdigest()
+        file_hash = file_metadata.get('file_hash') or await asyncio.to_thread(
+            lambda: hashlib.sha256(contents).hexdigest()
+        )
 
         file_item = await Files.insert_new_file(
             user.id,