新增 Pixiv token 刷新守护：每 30-60 分钟随机续期，refresh_token 轮换原子落盘

- refresh_token.py：启动即刷新，random 30~60min 循环，失败 5min 重试
- 每次保存新 access_token+refresh_token（Pixiv 会轮换），chmod 600
- pixiv-token-refresh.service：常驻守护，Restart=always
- README 同步服务清单与部署说明

systemd/pixiv-token-refresh.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Pixiv Token Refresh Daemon
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Environment=PATH=/home/openapi/env/general/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+Environment=SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt
+Type=simple
+WorkingDirectory=/home/openapi/source/pixiv
+ExecStart=/home/openapi/env/general/bin/python /home/openapi/source/pixiv/refresh_token.py
+Restart=always
+RestartSec=30
+StandardOutput=append:/var/log/pixiv_token_refresh.log
+StandardError=append:/var/log/pixiv_token_refresh.log
+
+[Install]
+WantedBy=multi-user.target