feat: add Atuin shell history synchronization service

- Add Atuin v18.10.0 service with PostgreSQL 14 database backend
- Configure Atuin server on port 4018 with open registration for demo
- Add PostgreSQL database service with proper health checks
- Update demo-stack.sh status display to include Atuin URL
- Update README.md service table with Atuin entry
- Add Atuin configuration variables to demo.env
- Use TCP socket health check for Atuin (no curl available in container)
- Atuin server accessible and responding with version info
- All services including Atuin now operational and healthy

SupportStack/demo/demo-stack.sh

@@ -113,6 +113,7 @@ show_status() {
     echo -e "${GREEN}   Atomic Tracker: http://localhost:${ATOMIC_TRACKER_PORT}${NC}"
     echo -e "${GREEN}   Wakapi: http://localhost:${WAKAPI_PORT}${NC}"
     echo -e "${GREEN}   MailHog: http://localhost:${MAILHOG_WEB_PORT}${NC}"
+    echo -e "${GREEN}   Atuin: http://localhost:${ATUIN_PORT}${NC}"
     echo -e "${GREEN}   Grafana: http://localhost:${GRAFANA_PORT}${NC}"
     echo -e "${GREEN}   Portainer: http://localhost:${PORTAINER_PORT}${NC}"
 }