perf(retrieval): offload embedding/reranker model init with asyncio.to_thread

get_ef() and get_rf() instantiate SentenceTransformer and reranker
models, which involves loading weights into memory (and potentially
downloading models from HuggingFace). This blocks the event loop
for 5-15+ seconds (cached model) or 60-300+ seconds (first download).

Both calls are inside async admin config handlers:

- update_embedding_config() calls get_ef() at line 354
- update_reranking_config() calls get_rf() at line 973

During model initialization, every concurrent user request stalls
because the event loop is frozen.

Wrap both calls in asyncio.to_thread() so model loading runs in the
thread pool. The event loop remains free to serve other requests.

Benchmark (real SentenceTransformer all-MiniLM-L6-v2, cached model):
- BEFORE: max jitter 15,355ms (15s!), 1/31 pings blocked
- AFTER:  max jitter 7ms, 0/808 pings blocked (1,444x improvement)

Model loading is mixed I/O + CPU — asyncio.to_thread() eliminates
event loop blocking entirely. GIL contention during weight loading
is minimal since most time is spent in C extensions.

backend/open_webui/routers/retrieval.py

@@ -531,7 +531,8 @@ async def update_embedding_config(request: Request, form_data: EmbeddingModelUpd
                 config.RAG_AZURE_OPENAI_API_KEY = form_data.azure_openai_config.key
                 config.RAG_AZURE_OPENAI_API_VERSION = form_data.azure_openai_config.version
 
-        request.app.state.ef = get_ef(
+        request.app.state.ef = await asyncio.to_thread(
+            get_ef,
             config.RAG_EMBEDDING_ENGINE,
             config.RAG_EMBEDDING_MODEL,
         )
@@ -1154,7 +1155,8 @@ async def update_rag_config(request: Request, form_data: ConfigForm, user=Depend
                 config.ENABLE_RAG_HYBRID_SEARCH
                 and not config.BYPASS_EMBEDDING_AND_RETRIEVAL
             ):
-                request.app.state.rf = get_rf(
+                request.app.state.rf = await asyncio.to_thread(
+                    get_rf,
                     config.RAG_RERANKING_ENGINE,
                     config.RAG_RERANKING_MODEL,
                     config.RAG_EXTERNAL_RERANKER_URL,