feat: add OSS offsite backup gate

deploy/server/install-offsite-backup.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+set -Eeuo pipefail
+
+if [[ ${EUID:-$(id -u)} -ne 0 ]]; then
+  echo "Run this installer as root" >&2
+  exit 2
+fi
+
+project_root=${PLATFORM_ROOT:-/opt/data-terminal-cloud/current}
+"$project_root/deploy/server/install-ossutil.sh"
+install -d -m 0700 /etc/data-terminal-cloud
+install -d -m 0750 /var/lib/data-terminal-cloud-offsite
+if [[ ! -e /etc/data-terminal-cloud/offsite-backup.env ]]; then
+  cat >/etc/data-terminal-cloud/offsite-backup.env <<'EOF'
+OFFSITE_BACKUP_ENABLED=false
+OFFSITE_BACKUP_REQUIRED=false
+OFFSITE_OSS_BUCKET=
+OFFSITE_OSS_PREFIX=data-terminal-cloud/121.41.46.238
+OFFSITE_OSS_CONFIG_FILE=/etc/data-terminal-cloud/ossutilconfig
+EOF
+fi
+chmod 0600 /etc/data-terminal-cloud/offsite-backup.env
+install -m 0644 "$project_root/deploy/server/platform-offsite-backup.service" /etc/systemd/system/platform-offsite-backup.service
+install -m 0644 "$project_root/deploy/server/platform-offsite-backup.timer" /etc/systemd/system/platform-offsite-backup.timer
+systemctl daemon-reload
+systemctl enable --now platform-offsite-backup.timer
+systemctl is-active --quiet platform-offsite-backup.timer
+echo "platform-offsite-backup.timer enabled; upload remains disabled until OSS identity and bucket are configured"