Saves an Expo push token for the authenticated user’s device.
Call this on every app launch after obtaining a push token from Expo’s
getPushTokenAsync(). The upsert is idempotent — re-registering the same
token is safe and simply refreshes is_active = true.
A user can have multiple active tokens (multiple devices). All active tokens receive push notifications for every escrow event.
JWT access token obtained from /auth-verify (login context) or /auth-signup. Set the bearer_token environment variable in your API client to apply it globally.