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/konto/bestellungen/[orderNumber]/page.tsx

@@ -108,6 +108,15 @@ export default async function KontoBestellungDetailPage({ params }: { params: Pr
             </div>
           </div>
 
+          {order.invoiceNumber && (
+            <a
+              href={`/api/account/orders/${encodeURIComponent(order.orderNumber)}/invoice`}
+              className="text-body-sm text-brand hover:underline"
+            >
+              Rechnung herunterladen ({order.invoiceNumber})
+            </a>
+          )}
+
           {action && <OrderActionButton orderNumber={order.orderNumber} action={action} />}
         </Reveal>
       </main>