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/payload.ts

@@ -577,13 +577,19 @@ export async function getShippingSettings(): Promise<ShippingSettings> {
   };
 }
 
-export type PaymentMethod = { id: number; title: string; icons: string[] };
+// `provider` drives the checkout branch in app/api/checkout/route.ts —
+// 'manual' (Überweisung) keeps today's immediate-order behavior, 'stripe'
+// (Kreditkarte/PayPal) routes through the payment-intent/webhook-gated
+// flow. Defaults to 'manual' below for any row created before this field
+// existed, matching the Payload field's own default.
+export type PaymentMethod = { id: number; title: string; icons: string[]; provider: "manual" | "stripe" };
 
 type PayloadPaymentMethod = {
   id: number;
   title: string;
   active: boolean;
   icons: { icon: { url: string } | number | null }[];
+  provider?: "manual" | "stripe";
 };
 
 export async function getPaymentMethods(): Promise<PaymentMethod[]> {
@@ -611,6 +617,7 @@ export async function getPaymentMethods(): Promise<PaymentMethod[]> {
     icons: (doc.icons ?? [])
       .map((row) => (typeof row.icon === "object" && row.icon ? row.icon.url : null))
       .filter((url): url is string => Boolean(url)),
+    provider: doc.provider ?? "manual",
   }));
 }