Buy traffic from the wallet balance (API and MCP)

Creates and pays an order for gb whole GB (or the one-off trial) from the wallet; the price comes from the tier table for the ordered quantity, the client never sends a price. accept_terms: true is required, as at checkout: it records that the customer accepts the terms and the acceptable-use policy and asks for the service to start at once (optional terms_version names the version shown). Requires an Idempotency-Key header: the same key with the same body replays the same order, the same key with a different body is 409 idempotency_conflict. With an API key a refused purchase is kept under its key for 24 h as well, so after fixing the cause (for example a top-up) send a new key. An API key needs the ports:write scope. 402 insufficient_funds when the wallet is too low, 402 kyb_required_for_volume above the monthly volume for unverified organisations, 403 account_suspended when traffic is paused on the account, 409 trial_already_used, 409 launch_waitlist while LAUNCH_MODE=waitlist.