fix: prevent worker death during document upload by using run_coroutine_threadsafe (#21158)

* fix: prevent worker death during document upload by using run_coroutine_threadsafe

Replace asyncio.run() with asyncio.run_coroutine_threadsafe() in
save_docs_to_vector_db() to prevent uvicorn worker health check failures.

The issue: asyncio.run() creates a new event loop and blocks the thread
completely, preventing the worker from responding to health checks during
long-running embedding operations (>5 seconds default timeout).

The fix: Schedule the async embedding work on the main event loop using
run_coroutine_threadsafe(). This keeps the main loop responsive to health
check pings while the sync caller waits for the result.

Changes:
- main.py: Store main event loop reference in app.state.main_loop at startup
- retrieval.py: Use run_coroutine_threadsafe() instead of asyncio.run()

https://claude.ai/code/session_01UQSYvSTkXb57sFb7M85Kcw

* add env var

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

backend/open_webui/main.py

@@ -591,6 +591,10 @@ https://github.com/open-webui/open-webui
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
+    # Store reference to main event loop for sync->async calls (e.g., embedding generation)
+    # This allows sync functions to schedule work on the main loop without blocking health checks
+    app.state.main_loop = asyncio.get_running_loop()
+
     app.state.instance_id = INSTANCE_ID
     start_logger()