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/lib/payload.ts

@@ -800,6 +800,14 @@ export type CompanySettings = {
   sellerEmail: string;
   vatId: string;
   taxRatePercent: number;
+  // Kleinunternehmerregelung (§19 UStG) — when true, checkout forces every
+  // order's items to 0% VAT (never de-grossed, unlike the intra-community
+  // exemption) and the tax rate above is ignored. Read live only at
+  // checkout time (see api/checkout/route.ts) to decide what to snapshot
+  // onto the new order — never read live when rendering an existing
+  // order's invoice, see OrderSnapshot/CustomerOrderDetail's own
+  // `kleinunternehmer` field for why.
+  kleinunternehmer: boolean;
   iban: string | null;
   bic: string | null;
 };
@@ -843,3 +851,22 @@ export async function getDefaultTaxRatePercent(): Promise<number> {
   const data: { docs?: { taxRatePercent: number }[] } = await res.json();
   return data.docs?.[0]?.taxRatePercent ?? 19;
 }
+
+// Same ISR-cached, public-catalog-freshness fetch as getDefaultTaxRatePercent()
+// above (a separate round trip rather than reusing getCompanySettings()'s
+// deliberate cache: "no-store") — powers the "inkl. X% MwSt." storefront
+// hints (dropped entirely when this is true, see ProductGrid.tsx/
+// ProductSpotlight.tsx/etc.) and the cart/checkout VAT-breakdown display.
+export async function getKleinunternehmer(): Promise<boolean> {
+  const params = new URLSearchParams({ "where[tenant.slug][equals]": TENANT_SLUG, limit: "1" });
+  const res = await fetch(`${PAYLOAD_URL}/api/company-settings?${params}`, {
+    headers: { "x-order-service-secret": process.env.ORDER_SERVICE_SECRET || "" },
+    next: { revalidate: 60 },
+  });
+  if (!res.ok) {
+    console.error(`getKleinunternehmer: Payload returned ${res.status} ${res.statusText}`);
+    return false;
+  }
+  const data: { docs?: { kleinunternehmer: boolean }[] } = await res.json();
+  return data.docs?.[0]?.kleinunternehmer ?? false;
+}