feat: sync — systemd unit and install.sh for server deployment

- verstak-server.service — systemd unit with sandboxing, configurable port via env

- install.sh — creates user, installs binary, sets up admin, enables service

  Usage: sudo ./install.sh --admin-user admin --admin-pass secret [--port 47732]

cmd/verstak-server/verstak-server.service

@@ -0,0 +1,22 @@
+[Unit]
+Description=Verstak Sync Server
+Documentation=https://github.com/anomalyco/verstak
+After=network.target
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/verstak-server --port ${VERSTAK_PORT} --data /var/lib/verstak-server
+Restart=on-failure
+RestartSec=5
+User=verstak
+Group=verstak
+AmbientCapabilities=CAP_NET_BIND_SERVICE
+NoNewPrivileges=true
+ProtectSystem=full
+ProtectHome=true
+PrivateTmp=true
+StateDirectory=verstak-server
+RuntimeDirectory=verstak-server
+
+[Install]
+WantedBy=multi-user.target