backupctl wraps a backup command: it pings /start, runs the command,
verifies the result, then pings /success or /fail. One static binary,
zero non-stdlib dependencies.
--url accepts a ping base (https://backpulse.aniicrite.dev, --key
required) or a full ping URL (.../ping/<key>[/start|/success|/fail]).
BACKPULSE_URL and BACKPULSE_PING_KEY are the flag defaults, so cron lines
stay short.