perf(backend): offload blocking calls in async paths to threads (#26381)

Audit of asyncio.sleep vs time.sleep and event-loop-blocking calls:

- utils/plugin.py: run pip `install_frontmatter_requirements`
  (subprocess.check_call) via asyncio.to_thread in load_tool_module_by_id,
  load_function_module_by_id, and install_tool_and_function_dependencies.
- retrieval/utils.py: move the synchronous SSRF-guarded requests probe and
  loader.load() in get_content_from_url into a sync helper run via
  asyncio.to_thread.
- routers/audio.py: write uploaded audio to disk off the event loop in
  transcription().
- routers/pipelines.py: write uploaded pipeline file off the event loop in
  upload_pipeline().

The existing time.sleep call sites are all in genuinely synchronous
functions (sync requests/DB drivers/daemon threads) with async
counterparts that already use asyncio.sleep, so no time.sleep -> asyncio.sleep
changes were needed.


Claude-Session: https://claude.ai/code/session_01LXR5bYfsfSS42RGHQZu2Ta

Co-authored-by: Claude <noreply@anthropic.com>

backend/open_webui/retrieval/utils.py

@@ -191,11 +191,17 @@ def _is_text_content_type(content_type: str) -> bool:
 
 
 async def get_content_from_url(request, url: str) -> str:
-    from open_webui.retrieval.web.utils import validate_url, _SSRFSafeAdapter
-
-
     loader_config = await get_loader_config()
 
+    # The rest of this function performs synchronous, blocking work: an SSRF-guarded
+    # `requests` probe and a synchronous document loader (`loader.load()`). Run it in a
+    # worker thread so the event loop stays free while waiting on network/parsing.
+    return await asyncio.to_thread(_get_content_from_url_sync, request, url, loader_config)
+
+
+def _get_content_from_url_sync(request, url: str, loader_config):
+    from open_webui.retrieval.web.utils import validate_url, _SSRFSafeAdapter
+
     # Validate URL before making any request (blocks private IPs, non-HTTP, filter list)
     validate_url(url)