Create a Stripe Checkout session for the authenticated company so a human can subscribe to Pro, monthly or yearly (optional interval month or year). Requires a company API key (aa_...) or an MCP OAuth access token. It does not accept a console session or a bootstrap token, and it does not mint a company or an aa_ secret. Returns url. Creating the session does not charge a card; the human pays on the Stripe page. When local billing bypass is enabled, there is no Stripe call and no charge: Pro is granted immediately with no period end, and url is the console success URL (/console?billing=ok), not a Stripe Checkout URL. If Stripe is not configured and bypass is off, the JSON includes error stripe_not_configured. For prepaid purchase instructions, use billing_machine_pay. To manage an existing subscription, use billing_portal.