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/lib/invoiceData.ts

@@ -0,0 +1,15 @@
+import { getInvoiceSettings } from "./payload";
+import { renderInvoicePdf, type InvoiceOrder } from "./invoicePdf";
+
+// Shared by app/lib/orderEmail.ts (checkout — attaches to the confirmation
+// mail) and app/api/account/orders/[orderNumber]/invoice/route.ts (on-demand
+// download) — same seller lookup + render call both times, so what a
+// customer downloads later always matches what they were emailed.
+export async function generateInvoicePdf(order: InvoiceOrder): Promise<Buffer | null> {
+  const seller = await getInvoiceSettings();
+  if (!seller) {
+    console.error("generateInvoicePdf: no invoice-settings row found for tenant");
+    return null;
+  }
+  return renderInvoicePdf(order, seller);
+}