chore(docker): add HEALTHCHECK probe

Python urllib GET / on 127.0.0.1:8000. start_period=30s
gives gunicorn workers time to boot. Lets Docker mark
container unhealthy if Flask app wedges, and feeds
Portainer + diagnose.sh signal that the app is alive.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/Dockerfile

@@ -11,4 +11,7 @@ COPY templates/ templates/
 ENV PYTHONUNBUFFERED=1
 EXPOSE 8000
 
+HEALTHCHECK --interval=30s --timeout=5s --retries=3 --start-period=30s \
+    CMD python -c "import urllib.request; urllib.request.urlopen('http://127.0.0.1:8000/').close()" || exit 1
+
 CMD ["gunicorn", "-b", "0.0.0.0:8000", "-w", "2", "--access-logfile", "-", "app:app"]