Called by the counterparty after clicking their WhatsApp invite link.
Adds them to the conversation and moves the escrow from invite_pending → active.
If the counterparty is a seller, provide payout_account_id so funds can be
released to their bank account when the escrow completes.
Both parties receive an active notification via the existing DB trigger.
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.