feat: change agent installer to use restart instead of enable --now for systemd service activation

Replace `systemctl enable --now` with separate `systemctl enable` and `systemctl restart` commands to ensure agent service restarts on reinstall rather than silently failing when service already exists

backend/app/api/v1/router.py

@@ -730,7 +730,8 @@ EOF
 
 if command -v systemctl >/dev/null 2>&1; then
   systemctl daemon-reload
-  systemctl enable --now "$SERVICE_NAME.service"
+  systemctl enable "$SERVICE_NAME.service"
+  systemctl restart "$SERVICE_NAME.service"
   systemctl status "$SERVICE_NAME.service" --no-pager || true
 else
   echo "systemctl not found. Run manually: python3 $INSTALL_DIR/nexafabric-agent.py --config $CONFIG_DIR/config.json"