The sync generator was failing with a network error because Starlette's
iterate_in_threadpool doesn't properly propagate exceptions from sync
generators — the connection resets with no body.
Fix: convert event_stream to an async generator (Starlette handles these
natively without thread wrapping), move imports to the endpoint function
scope so failures raise HTTPException before the stream starts, and run
CPU-intensive work (parse + write) via loop.run_in_executor so the
async generator can actually yield between activities.
Davide Scaini
10dd1185b9