Start an account-bound Stripe Checkout Session

Browser endpoint. Requires an active Fonteum developer-session cookie, resolves the signed-in developer account's Stripe Customer, and creates a hosted Checkout Session bound with client_reference_id and account/plan metadata. The returned Stripe URL ultimately redirects to /checkout/success?session_id={CHECKOUT_SESSION_ID}. Self-serve products are paid Line-B API tiers and the one-time $129 signed certificate; recurring Line-A monitoring is not published or accepted until its fulfillment exists. This endpoint is not authenticated by the Bearer API key.