Add Kleinunternehmerregelung (§19 UStG) support end-to-end

Checkout forces 0% VAT without de-grossing prices when the tenant is a
Kleinunternehmer (a business decision, not just an engineering default —
unlike the existing intra-community VAT exemption, which does de-gross).
Snapshotted onto the order at checkout time so a later toggle of the
company-settings checkbox never rewrites an already-issued invoice's tax
treatment — same reasoning as the existing vatExempt field.

Threaded through: checkout route, order creation/confirmation email,
on-demand invoice/Storno/Gutschrift downloads, the Bestellbestätigung
page, and the account order-detail page. The four storefront "inkl. X%
MwSt." price hints (shop grid, cart upsell, ToDo-Karten landing page,
homepage spotlight) drop that clause live when the setting is on. The
company-settings Live Preview reflects the checkbox in real time too.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/api/checkout/route.ts

@@ -141,6 +141,15 @@ export async function POST(request: Request) {
   // Re-price everything server-side — never trust client-submitted prices.
   const [productsBySlug, companySettings] = await Promise.all([fetchProductsBySlug(), getCompanySettings()]);
   const defaultTaxRate = companySettings?.taxRatePercent ?? 19;
+  // §19 UStG — a Kleinunternehmer tenant never charges VAT on anything,
+  // full stop, so every item's tax rate is forced to 0% here regardless of
+  // its own catalog/company-settings default rate. Unlike the
+  // intra-community exemption below, prices are NOT de-grossed — see
+  // Orders.ts's own kleinunternehmer field comment and this shop's
+  // Kleinunternehmer decision: catalog gross prices stay exactly what they
+  // are, they simply never had a VAT component charged on top in the
+  // first place.
+  const kleinunternehmer = Boolean(companySettings?.kleinunternehmer);
   const items: {
     productId: number;
     productName: string;
@@ -182,7 +191,7 @@ export async function POST(request: Request) {
       quantity: line.qty,
       unitPrice: variant?.priceOverride ?? product.price,
       imageUrl,
-      taxRatePercent: product.taxRatePercent ?? defaultTaxRate,
+      taxRatePercent: kleinunternehmer ? 0 : (product.taxRatePercent ?? defaultTaxRate),
       bundleContents: describeBundleContents(product),
       variantName: variant?.name ?? null,
     });
@@ -227,8 +236,13 @@ export async function POST(request: Request) {
   // unset in that case too).
   let vatExempt = false;
   let vatIdValidatedAt: string | null = null;
+  // A Kleinunternehmer never charges VAT on any sale, domestic or
+  // cross-border — the intra-community exemption exists to zero-rate what
+  // would otherwise be a positive-rate charge, which never applies here in
+  // the first place, so the VIES lookup is skipped entirely (also saves an
+  // unneeded network round-trip).
   const buyerDestinationCountry = destinationCountry(body.country, Boolean(body.hasDifferentShippingAddress), body.shippingCountry);
-  if (normalizedVatId) {
+  if (!kleinunternehmer && normalizedVatId) {
     const viesResult = await checkVatIdViaVies(normalizedVatId);
     if (viesResult.ok && viesResult.valid) {
       vatIdValidatedAt = new Date().toISOString();
@@ -275,6 +289,7 @@ export async function POST(request: Request) {
     companyName: body.companyName || undefined,
     vatId: normalizedVatId,
     vatExempt,
+    kleinunternehmer,
     vatIdValidatedAt,
     deliveryMethod: body.deliveryMethod,
     street: body.street,
@@ -335,6 +350,7 @@ export async function POST(request: Request) {
       companyName: body.companyName || undefined,
       vatId: normalizedVatId,
       vatExempt,
+      kleinunternehmer,
       deliveryMethod: body.deliveryMethod,
       street: body.street,
       packstationNumber: body.packstationNumber,
@@ -393,5 +409,6 @@ export async function POST(request: Request) {
     discountCode: body.discountCode || null,
     discountAmount,
     vatExempt,
+    kleinunternehmer,
   });
 }