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/api/answer/test_trace_wiring.py

@@ -22,7 +22,7 @@ def _captured_flushes():
     """Record every flushed trace instead of writing it."""
     flushed = []
 
-    def _fake_flush(trace, status=None):
+    def _fake_flush(trace, status=None, **_kwargs):
         if trace is None or trace.flushed:
             return
         trace.flushed = True