Create a Stripe PaymentIntent to buy mu by dollar amount at the peg

Creates a Stripe PaymentIntent for the authenticated caller's organization to buy mu at the fixed peg (1 mu = 1¢, face-only retail — MNE-3255). Owner/admin only (org_id query targets a specific org; a non-member or under-privileged caller is denied 403). api resolves the org's billing_account_id server-side and authors the upstream body itself — a client-supplied account reference is never forwarded. amountUsdCents must be a JSON integer; the min/max purchase bounds are enforced by mnemom-billing (its 400 passes back verbatim). idempotencyKey is REQUIRED — billing scopes the Stripe idempotency key to it so a retry of the same purchase cannot double-charge. Response is mnemom-billing's verbatim {paymentIntentId, clientSecret, expectedMmu} on 201; expectedMmu is a display prediction, never mint authority — the ledger credits only when payment_intent.succeeded arrives on the webhook.