feat(infra) : add maintenance mode support to production nginx config

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

infra/prod/Dockerfile

@@ -63,6 +63,7 @@ RUN rm -f /etc/nginx/sites-enabled/default
 # Configs
 COPY infra/prod/supervisord.conf /etc/supervisor/conf.d/app.conf
 COPY infra/prod/nginx.conf /etc/nginx/sites-enabled/inventory.conf
+COPY infra/prod/maintenance.html /var/www/html/public/maintenance.html
 
 # Backend from stage 1
 COPY --from=backend-build /app /var/www/html