Create Mandate Setup

Starts a Direct Debit setup with GoCardless through Account Hub, making the billing account on first use. Send the customer's browser to the returned redirectUrl; GoCardless sends it back to the successRedirectUrl with redirect_flow_id appended, and the client completes the setup with PUT /me/billing/mandate. 400 when the caller already has a mandate or GoCardless refused; 409 when the account also pays for Sodium.