Generate invoice PDFs attached to order confirmation, and send emails on order status changes

Invoice PDFs (§14 UStG line items, tenant-configurable VAT rate) are now
generated at checkout and attached to the confirmation email, plus
available on demand from the order-detail page. Payload-side, orders now
also email the customer on shipped/cancelled/return_requested/returned,
with Stornorechnung/Gutschrift correction PDFs attached for the latter two
so the original invoice's immutable number stays honest.

app/api/checkout/route.ts

@@ -88,11 +88,12 @@ 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 }[] = [];
+  const items: { productId: number; productName: string; quantity: number; unitPrice: number; imageUrl: 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 });
-    items.push({ productId: product.id, productName: product.name, quantity: line.qty, unitPrice: product.price });
+    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 });
   }
   const subtotal = items.reduce((sum, i) => sum + i.quantity * i.unitPrice, 0);
 
@@ -164,7 +165,18 @@ export async function POST(request: Request) {
     {
       orderNumber: order.orderNumber,
       createdAt: order.createdAt,
-      items: items.map((i) => ({ productName: i.productName, quantity: i.quantity, unitPrice: i.unitPrice })),
+      invoiceNumber: order.invoiceNumber,
+      invoiceIssuedAt: order.invoiceIssuedAt,
+      customerFirstName: body.firstName,
+      customerLastName: body.lastName,
+      deliveryMethod: body.deliveryMethod,
+      street: body.street,
+      packstationNumber: body.packstationNumber,
+      postNumber: body.postNumber,
+      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 })),
       subtotal,
       shippingCost,
       discountAmount,