Write chat traces off the stream's thread

The OTel replay and the trace INSERT ran in the stream's finally, so a slow
database held the SSE connection open after the last event. The trace is
still frozen when the stream ends, but written on a small writer pool.

tests/worker/test_extract_graph.py

@@ -188,7 +188,7 @@ class TestExtractGraphTrace:
         monkeypatch.setattr("docsgpt.graphrag.extraction.extract_graph_for_source", extract)
         flushed = []
 
-        def _flush(trace, status=None):
+        def _flush(trace, status=None, **_kwargs):
             trace.flushed = True
             trace.finish(status)
             flushed.append(trace)