- TONNODE_KEYS_FILE: JSON key registry with per-key rpm, expiry dates and
  labels; hot-reloaded on change and SIGHUP — add/revoke without restart,
  revoked keys lose live sessions immediately
- GLOBAL_RATE_LIMIT_RPM: total ceiling across all keys protecting the
  backend liteserver
- a keys file emptied at runtime locks the server instead of opening it
- deploy/tonnode-keys.sh: add/revoke/list helper (key shown once, expiry
  in days for subscription plans)
- systemd unit: fixed service user instead of DynamicUser so the 600-mode
  keys file is readable; ExecReload sends SIGHUP