Create a Monero or Zcash view-key payment watch. The $0.10 fee IS your starting credit (nothing is lost to a creation charge). Body: { chain, address, viewKey, webhookUrl, birthdayHeight? }. Returns { watchId, watchToken, webhookSecret, expiresAt, creditAtomic }; verify webhooks with HMAC-SHA256(webhookSecret, body), top up via /v1/private/topup*. Prefer no USDC? POST /v1/private/watch-crypto (free call) creates + funds the watch in XMR/ZEC.