Redesign invoice PDFs, add correction-invoice downloads, return reasons, and per-product tax/bundle support

Invoice + Stornorechnung/Gutschrift PDFs get a modern header-band layout,
a "bereits beglichen" badge for immediately-paid orders, labelled bank
details, and a per-tax-rate summary breakdown. Correction invoices can
now be re-downloaded from the account (regenerated deterministically,
not stored as files, same approach as the original invoice). Return
requests capture a reason. Products can define bundles (bundleItems) and
a per-product VAT rate override, both snapshotted onto order items.

app/api/checkout/route.ts

@@ -1,13 +1,28 @@
 import { NextResponse } from "next/server";
 import type { CartItem } from "../../lib/cart";
-import { getShippingMethods, getPaymentMethods } from "../../lib/payload";
+import { getShippingMethods, getPaymentMethods, getInvoiceSettings } from "../../lib/payload";
 import { validateDiscountCode, redeemDiscountCode } from "../../lib/discountServer";
 import { createOrder } from "../../lib/orderServer";
 import { getSessionCustomer, registerCustomer, setSessionCookie, type CustomerSummary } from "../../lib/customerAuth";
-import { fetchProductsBySlug } from "../../lib/productsServer";
+import { fetchProductsBySlug, type RawProduct } from "../../lib/productsServer";
 import { sendCriticalAlert } from "../../lib/alertAdmin";
 import { sendOrderConfirmationEmail } from "../../lib/orderEmail";
 
+// A bundle's `bundleItems` is only ever resolved once, right here, into a
+// plain readable string — the order line stores this snapshot
+// (`items[].bundleContents`), not a structured sub-list, so a later change
+// to the bundle's own composition can never rewrite what a past order
+// actually contained. `product.bundleItems[].product` is a relationship
+// resolved by fetchProductsBySlug's depth:2 fetch — {id, name} objects
+// when populated, a bare id if depth somehow didn't reach it (skipped).
+function describeBundleContents(product: RawProduct): string | null {
+  if (!product.bundleItems || product.bundleItems.length === 0) return null;
+  return product.bundleItems
+    .map((line) => (typeof line.product === "object" ? `${line.quantity}× ${line.product.name}` : null))
+    .filter((s): s is string => Boolean(s))
+    .join(", ");
+}
+
 type CheckoutBody = {
   cart: CartItem[];
   shippingMethodId: number;
@@ -87,13 +102,30 @@ export async function POST(request: Request) {
   }
 
   // Re-price everything server-side — never trust client-submitted prices.
-  const productsBySlug = await fetchProductsBySlug();
-  const items: { productId: number; productName: string; quantity: number; unitPrice: number; imageUrl: string | null }[] = [];
+  const [productsBySlug, invoiceSettings] = await Promise.all([fetchProductsBySlug(), getInvoiceSettings()]);
+  const defaultTaxRate = invoiceSettings?.taxRatePercent ?? 19;
+  const items: {
+    productId: number;
+    productName: string;
+    quantity: number;
+    unitPrice: number;
+    imageUrl: string | null;
+    taxRatePercent: number;
+    bundleContents: string | null;
+  }[] = [];
   for (const line of body.cart) {
     const product = productsBySlug.get(line.id);
     if (!product) return NextResponse.json({ ok: false, reason: "Ein Artikel im Warenkorb ist nicht mehr verfügbar." }, { status: 400 });
     const imageUrl = typeof product.image === "object" && product.image ? product.image.url : null;
-    items.push({ productId: product.id, productName: product.name, quantity: line.qty, unitPrice: product.price, imageUrl });
+    items.push({
+      productId: product.id,
+      productName: product.name,
+      quantity: line.qty,
+      unitPrice: product.price,
+      imageUrl,
+      taxRatePercent: product.taxRatePercent ?? defaultTaxRate,
+      bundleContents: describeBundleContents(product),
+    });
   }
   const subtotal = items.reduce((sum, i) => sum + i.quantity * i.unitPrice, 0);
 
@@ -176,7 +208,15 @@ export async function POST(request: Request) {
       zip: body.zip,
       city: body.city,
       country: body.country,
-      items: items.map((i) => ({ productName: i.productName, quantity: i.quantity, unitPrice: i.unitPrice, imageUrl: i.imageUrl })),
+      paymentMethodTitle: paymentMethod.title,
+      items: items.map((i) => ({
+        productName: i.productName,
+        quantity: i.quantity,
+        unitPrice: i.unitPrice,
+        imageUrl: i.imageUrl,
+        taxRatePercent: i.taxRatePercent,
+        bundleContents: i.bundleContents,
+      })),
       subtotal,
       shippingCost,
       discountAmount,