fix(docker): set explicit FRONTEND_ORIGIN env var (BUG-009)

Add FRONTEND_ORIGIN environment variable to Dockerfile as a backup
to the CORS regex, making the allowed origin explicit:
https://vibecodermcswaggins-stroke-viewer-frontend.hf.space

Dockerfile

@@ -75,6 +75,9 @@ EXPOSE 7860
 # Reset ENTRYPOINT from base image
 ENTRYPOINT []
 
+# Explicit frontend origin for CORS (backup to regex)
+ENV FRONTEND_ORIGIN=https://vibecodermcswaggins-stroke-viewer-frontend.hf.space
+
 # Run FastAPI with uvicorn (module path: stroke_deepisles_demo.api.main:app)
 # --proxy-headers: Trust X-Forwarded-Proto from HF Spaces proxy (ensures https:// in request.base_url)
 CMD ["uvicorn", "stroke_deepisles_demo.api.main:app", "--host", "0.0.0.0", "--port", "7860", "--proxy-headers"]