Dockerfile: gunicorn 2 workers x 4 threads (gthread)

Single sync worker means a slow request to / (compute_leaderboard, ~17s
on dev / longer on h4a's compact CPU slice) blocks every other request,
including the /healthz probe. With 2 worker processes and 4 threads
each, slow requests on / no longer wedge the gunicorn queue, and
/healthz responds promptly even while a /api/calculate is in flight.

Dockerfile

@@ -24,5 +24,7 @@ EXPOSE 8080
 
 CMD ["gunicorn", "pluginbattery.web:app", \
      "--bind", "0.0.0.0:8080", \
-     "--workers", "1", \
+     "--workers", "2", \
+     "--threads", "4", \
+     "--worker-class", "gthread", \
      "--timeout", "120"]