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/customerAuth.ts

@@ -424,6 +424,8 @@ export async function getCustomerOrders(token: string, customerId: number): Prom
 
 export type CustomerOrderDetail = CustomerOrder & {
   id: number;
+  invoiceNumber: string | null;
+  invoiceIssuedAt: string | null;
   customerFirstName: string;
   customerLastName: string;
   customerEmail: string;