systemd installation

Install the application files under /opt/rustpad and the binary as /usr/local/bin/rustpad.
Create the service account: useradd --system --home /var/lib/rustpad --shell /usr/sbin/nologin rustpad.
Copy rustpad.yaml to /etc/rustpad/rustpad.yaml and restrict secrets: chmod 640 /etc/rustpad/rustpad.yaml.
Copy rustpad.service to /etc/systemd/system/rustpad.service.
Validate before starting: /usr/local/bin/rustpad --config /etc/rustpad/rustpad.yaml check-config.
Run systemctl daemon-reload && systemctl enable --now rustpad.

Environment variables and an optional .env file override the YAML values. This allows secrets to be supplied by the service manager without editing the main configuration file.