Threaded gunicorn workers so a retry storm cannot starve the UI

Dockerfile

@@ -139,10 +139,16 @@ HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
         -H "Content-Type: application/json" -d '{}' \
         | grep -q "VALIDATION_ERROR" || exit 1
 
+# Threaded workers, matching docker-compose.yml - see the reasoning there. A
+# compose file overrides this CMD, but an image run without one (a plain
+# `docker run`, a site's own orchestrator) would otherwise still get the
+# blocking sync default and the same starvation.
 CMD ["gunicorn", \
      "--bind", "0.0.0.0:5001", \
+     "--worker-class", "gthread", \
      "--workers", "4", \
-     "--timeout", "60", \
+     "--threads", "8", \
+     "--timeout", "30", \
      "--access-logfile", "-", \
      "--error-logfile", "-", \
      "wsgi:app"]