feat: 增量升级路径 --upgrade / --ui-only

- 已装机器默认自动走升级，保留 secret/网卡/账密
- 默认不重下 sing-box、不 rebind、不 verify
- scripts/upgrade.sh + install.sh 入口 + README

systemd/cellular-proxy-admin.service

@@ -0,0 +1,24 @@
+[Unit]
+Description=cellular-proxy admin API (proxy auth config)
+After=network-online.target cellular-proxy.service
+Wants=network-online.target
+
+[Service]
+Type=simple
+Environment=INSTALL_DIR=@INSTALL_DIR@
+Environment=CPXY_SETTINGS=@INSTALL_DIR@/etc/settings.conf
+Environment=CPXY_APPLY_AUTH=@INSTALL_DIR@/scripts/apply-proxy-auth.sh
+Environment=ADMIN_LISTEN_HOST=0.0.0.0
+Environment=ADMIN_LISTEN_PORT=9091
+ExecStart=/usr/bin/python3 @INSTALL_DIR@/scripts/admin-api.py
+Restart=on-failure
+RestartSec=2
+NoNewPrivileges=true
+ProtectSystem=full
+ProtectHome=true
+PrivateTmp=true
+ReadWritePaths=@INSTALL_DIR@ /var/lib/cellular-proxy /var/log/cellular-proxy
+User=root
+
+[Install]
+WantedBy=multi-user.target