_send_webhook:
- Dedupe: skip if alert.webhook_sent is already True (guards against
repeated check_alerts ticks re-sending the same alert).
- Retry: up to 3 attempts with exponential backoff (1s, 2s) before
giving up. webhook_sent=True is persisted only on success.
Two xfail guards in test_openclaw_client_and_webhook now pass (876/4 xfail).