Create session

Creates a Stripe checkout session for a subscription against the Stripe product+price chosen by the frontend. Stripe rejects unknown/archived/mode-mismatched price IDs. Returns 409 use_create_customer_portal_session if the org already has an active subscription in Stripe (source of truth -- bypasses any JWT staleness). FE should route those users to POST /billing/create-customer-portal-session/v1 to manage. authz: allowed_org_types=[provider] (shippers cannot subscribe -- they are never gated, paying would be a no-op), min_org_role=administrator (committing the company to recurring charges is a finance decision, not an operator one). | (CreateCheckoutSessionReq) -> (CreateCheckoutSessionRes)