Add Kleinunternehmerregelung (§19 UStG) support

InvoiceOrder/CorrectionInvoiceOrder gain an optional order-level
kleinunternehmer flag, snapshotted per order (same pattern as vatExempt)
so a later toggle of the tenant's setting never rewrites an
already-issued invoice. Takes precedence over vatExempt.

- Visual PDF: "enthält X% MwSt." becomes the §19 UStG notice.
- E-invoice XML: new UNTDID 5305 category "E" with a free-text exemption
  reason (no VATEX code — §19 UStG has none, BR-E-10 allows text alone).
- New Mustang CI fixture + buildEInvoiceData unit test for the "E" path.

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

scripts/generate-einvoice-fixtures.mts

@@ -85,6 +85,35 @@ const CORRECTION_ORDER: CorrectionInvoiceOrder = {
   total: ORDER.total,
 };
 
+// §19 UStG Kleinunternehmer — every item snapshotted at 0% (exactly what
+// api/checkout/route.ts forces when the tenant has company-settings.
+// kleinunternehmer set), UNTDID 5305 category "E" with a free-text
+// exemption reason instead of "S"/percent-rated lines. Own fixture rather
+// than a variant of ORDER above, since a real Kleinunternehmer order never
+// has a positive rate to begin with — mixing that into the 19%/7% fixture
+// would test a combination (kleinunternehmer + a nonzero catalog rate)
+// that can't actually happen in production.
+const KLEINUNTERNEHMER_ORDER: InvoiceOrder = {
+  orderNumber: "#EP-MUSTANG-0002",
+  invoiceNumber: "RE-MUSTANG-0002",
+  invoiceIssuedAt: new Date().toISOString(),
+  customerFirstName: "Erika",
+  customerLastName: "Musterfrau",
+  deliveryMethod: "address",
+  street: "Kundenweg 2",
+  zip: "80331",
+  city: "München",
+  country: "Deutschland",
+  paymentMethodTitle: "Kreditkarte",
+  kleinunternehmer: true,
+  items: [{ productName: "ToDo-Karten – Set", quantity: 2, unitPrice: 12.9, taxRatePercent: 0, bundleContents: null }],
+  subtotal: 25.8,
+  shippingCost: 4.95,
+  discountAmount: 0,
+  discountCode: null,
+  total: 30.75,
+};
+
 async function main() {
   await mkdir(OUT_DIR, { recursive: true });
 
@@ -100,7 +129,10 @@ async function main() {
   const gutschrift = await renderCorrectionInvoiceEInvoice("gutschrift", CORRECTION_ORDER, SELLER);
   await writeFile(new URL("gutschrift.pdf", OUT_DIR), gutschrift);
 
-  console.log(`Wrote 3 e-invoice fixtures to ${OUT_DIR.pathname}`);
+  const kleinunternehmerInvoice = await renderInvoiceEInvoice(KLEINUNTERNEHMER_ORDER, SELLER);
+  await writeFile(new URL("kleinunternehmer-invoice.pdf", OUT_DIR), kleinunternehmerInvoice);
+
+  console.log(`Wrote 4 e-invoice fixtures to ${OUT_DIR.pathname}`);
 }
 
 main().catch((err) => {