Add Stripe payment processing (cards + PayPal) with a webhook-gated checkout flow

Checkout now branches on payment-methods.provider: Überweisung stays
immediate/unchanged, Kreditkarte/PayPal creates a pending_payment order,
mounts Stripe's Payment Element, and defers invoice/email to a webhook-
verified confirm-payment call once the backend actually confirms payment.
Includes a PAYMENT_TEST_MODE mock provider so the whole gated pipeline is
exercisable locally without a real Stripe account.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/customerAuth.ts

@@ -443,6 +443,10 @@ export async function getCustomerOrders(token: string, customerId: number): Prom
 
 export type CustomerOrderDetail = CustomerOrder & {
   id: number;
+  // 'not_applicable' for Überweisung orders (never gated); see
+  // spicy-leaping-pizza.md §1 — read by /api/checkout/status for the
+  // post-Stripe-redirect polling page.
+  paymentStatus: "not_applicable" | "pending" | "paid" | "failed" | "refunded" | "partially_refunded";
   invoiceNumber: string | null;
   invoiceIssuedAt: string | null;
   correctionInvoiceNumber: string | null;