Customer Pay Existing Transaction

Initiate a cashier session for an existing PENDING transaction.

Used by the webtrader to pay an already-created CHALLENGE_FEE / CHALLENGE_RESET_FEE / CHALLENGE_ADDON_FEE transaction (created at grant time by CRM or at self-enroll time). The tx already carries amount/currency/account_id/journey_id — this endpoint stamps a cashier_id, calls integration-service to create a payment session, and returns the iframe/redirect config for the frontend to render.

The webhook handler (payment_callback) is unchanged: when the cashier APPROVES the tx it dispatches to activate_prop_account / apply_addon_to_journey based on tx.type. No money is moved here.