Blog categories (hasMany), shipping-address contact fields, product SKU on invoices

- Posts.categories is now hasMany — blog list/detail/live-preview render
  a comma-joined list instead of a single category.
- Checkout's "Abweichende Lieferadresse" gains optional Firma + Kontakt-
  E-Mail/Telefon fields (handed to the shipping carrier, not used for
  customer communication).
- Customer profile can now store its own shipping address (mirroring
  Customers.ts's new "Lieferadresse" tab), prefilling the checkout
  override instead of always starting blank.
- Product-level optional SKU (previously only on variants) snapshots onto
  each order item and shows up on invoices (visual + EN16931 XML),
  the confirmation email, and the account order detail page.

app/api/checkout/route.ts

@@ -54,6 +54,9 @@ type CheckoutBody = {
   shippingZip?: string;
   shippingCity?: string;
   shippingCountry?: string;
+  shippingCompanyName?: string;
+  shippingContactEmail?: string;
+  shippingContactPhone?: string;
   newsletterOptIn: boolean;
 };
 
@@ -160,6 +163,7 @@ export async function POST(request: Request) {
     taxRatePercent: number;
     bundleContents: string | null;
     variantName: string | null;
+    sku: string | null;
   }[] = [];
   for (const line of body.cart) {
     const product = productsBySlug.get(line.id);
@@ -168,7 +172,7 @@ export async function POST(request: Request) {
     // requested variant that no longer exists on this product (removed,
     // or never existed — a tampered request) fails the whole checkout
     // rather than silently falling back to the base product/price.
-    let variant: { name: string; priceOverride: number | null } | null = null;
+    let variant: { name: string; priceOverride: number | null; sku: string | null } | null = null;
     if (line.variant) {
       variant = product.variants?.find((v) => v.name === line.variant) ?? null;
       if (!variant) return NextResponse.json({ ok: false, reason: "Eine gewählte Variante ist nicht mehr verfügbar." }, { status: 400 });
@@ -195,6 +199,9 @@ export async function POST(request: Request) {
       taxRatePercent: kleinunternehmer ? 0 : (product.taxRatePercent ?? defaultTaxRate),
       bundleContents: describeBundleContents(product),
       variantName: variant?.name ?? null,
+      // Variant sku takes precedence over the product's own — same
+      // "variant overrides product" precedence unitPrice already uses.
+      sku: variant?.sku ?? product.sku ?? null,
     });
   }
   const subtotal = roundMoney(items.reduce((sum, i) => sum + i.quantity * i.unitPrice, 0));
@@ -345,11 +352,15 @@ export async function POST(request: Request) {
     shippingZip: body.shippingZip,
     shippingCity: body.shippingCity,
     shippingCountry: body.shippingCountry,
+    shippingCompanyName: Boolean(body.hasDifferentShippingAddress) ? body.shippingCompanyName || undefined : undefined,
+    shippingContactEmail: Boolean(body.hasDifferentShippingAddress) ? body.shippingContactEmail || undefined : undefined,
+    shippingContactPhone: Boolean(body.hasDifferentShippingAddress) ? body.shippingContactPhone || undefined : undefined,
     newsletterOptIn: Boolean(body.newsletterOptIn),
     items,
     subtotal: finalSubtotal,
     shippingCost: finalShippingCost,
     shippingMethodTitle: shippingMethod.title,
+    shippingMethod: shippingMethod.id,
     // The checkout UI collapses Kreditkarte/PayPal into one "Online-
     // Zahlung" pre-selection (see groupPaymentMethodsForCheckout) — the
     // customer hasn't actually chosen an instrument yet at this point,
@@ -429,6 +440,7 @@ export async function POST(request: Request) {
         hasDifferentShippingAddress: Boolean(body.hasDifferentShippingAddress),
         shippingFirstName: body.shippingFirstName,
         shippingLastName: body.shippingLastName,
+        shippingCompanyName: body.hasDifferentShippingAddress ? body.shippingCompanyName : undefined,
         shippingDeliveryMethod: body.shippingDeliveryMethod,
         shippingStreet: body.shippingStreet,
         shippingPackstationNumber: body.shippingPackstationNumber,
@@ -436,6 +448,8 @@ export async function POST(request: Request) {
         shippingZip: body.shippingZip,
         shippingCity: body.shippingCity,
         shippingCountry: body.shippingCountry,
+        shippingContactEmail: body.hasDifferentShippingAddress ? body.shippingContactEmail : undefined,
+        shippingContactPhone: body.hasDifferentShippingAddress ? body.shippingContactPhone : undefined,
         paymentMethodTitle: paymentMethod.title,
         items: items.map((i) => ({
           productName: i.productName,
@@ -445,6 +459,7 @@ export async function POST(request: Request) {
           taxRatePercent: i.taxRatePercent,
           bundleContents: i.bundleContents,
           variantName: i.variantName,
+          sku: i.sku,
         })),
         subtotal: finalSubtotal,
         shippingCost: finalShippingCost,