Mint a Stripe Customer Session for the caller's org (saved-method redisplay)

Mints a short-lived Stripe Customer Session client secret for the authenticated caller's organization. Passed into stripe.elements() as customerSessionClientSecret, it lets the Payment Element redisplay the org's saved payment methods and offer the save-for-future checkbox — the modern returning-buyer flow. Creates a Stripe object only: no charge, no ledger movement. Owner/admin only, same org resolution as the purchase route. Takes NO request body — any body sent is ignored; api authors the upstream request entirely from the server-resolved billing_account_id. The secret expires roughly 30 minutes after minting, so request a fresh one per Payment Element mount. Response is mnemom-billing's verbatim {stripeCustomerId, customerSessionClientSecret} on 201.