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>

src/einvoice/buildEInvoiceData.ts

@@ -23,17 +23,33 @@ type BuyerCountryCode = UblInvoice["cac:AccountingCustomerParty"]["cac:Party"]["
 // summed group uses this same code.
 const VAT_CATEGORY = "S";
 
-// UNTDID 5305 code 'K' — "VAT exempt for EEA intra-community supply of
-// goods and services" — the EN16931 category for an innergemeinschaftliche
-// Lieferung (§4 Nr. 1b UStG / Art. 138 VAT Directive), distinct from 'S'
-// (positive standard rate), 'Z' (zero-rated but still taxable), and 'AE'
-// (domestic reverse charge under §13b UStG — doesn't apply here at all,
-// see the frontend repo's lib/vatExemption.ts for why this shop's checkout
-// only ever produces this 'K' exemption for a cross-border validated B2B
-// sale, never 'AE').
-function vatTaxCategory(rate: number, vatExempt: boolean): Record<string, unknown> {
+// Which of this shop's three mutually-exclusive VAT treatments an
+// invoice/correction-invoice was issued under — 'standard' (normal
+// Regelbesteuerung), 'intra-community' (validated cross-border B2B
+// exemption, §4 Nr. 1b UStG), or 'kleinunternehmer' (§19 UStG small
+// business — the seller never charges VAT at all, on any sale). Derived
+// once per document from the order's own snapshotted
+// vatExempt/kleinunternehmer flags (see buildEInvoiceData/
+// buildCorrectionEInvoiceData below) and threaded through every function
+// in this file that used to take a plain `vatExempt: boolean` — a
+// Kleinunternehmer sale is never also an intra-community exemption
+// (there's no VAT to exempt in the first place), so this is a 3-way
+// choice, not two independent booleans.
+type VatMode = "standard" | "intra-community" | "kleinunternehmer";
+
+// UNTDID 5305 codes: 'K' — "VAT exempt for EEA intra-community supply of
+// goods and services" (§4 Nr. 1b UStG / Art. 138 VAT Directive). 'E' —
+// "Exempt from tax", used here for §19 UStG Kleinunternehmer — Germany's
+// small-business exemption is a national provision, not one of the
+// EU-wide reasons that has its own dedicated UNTDID/VATEX category, so
+// 'E' (the general exemption code) plus a free-text reason (BT-120, see
+// vatTaxSubtotalCategory below) is the correct EN16931 shape rather than
+// forcing it into 'K'. Distinct from 'S' (positive standard rate) and 'Z'
+// (zero-rated but still taxable) — neither applies to either exemption.
+function vatTaxCategory(rate: number, vatMode: VatMode): Record<string, unknown> {
+  const id = vatMode === "kleinunternehmer" ? "E" : vatMode === "intra-community" ? "K" : VAT_CATEGORY;
   return {
-    "cbc:ID": vatExempt ? "K" : VAT_CATEGORY,
+    "cbc:ID": id,
     "cbc:Percent": String(rate),
     "cac:TaxScheme": { "cbc:ID": "VAT" },
   };
@@ -48,13 +64,17 @@ function vatTaxCategory(rate: number, vatExempt: boolean): Record<string, unknow
 // generated type despite conceptually being "the same" UBL TaxCategory
 // complex type) — caught locally before ever reaching Mustang/CI, exactly
 // the kind of runtime-only-visible constraint this pipeline has hit before
-// (see amtCents()'s own comment). BR-K-10 requires either field whenever
-// category is 'K', satisfied here since this is the one place they're
-// actually allowed to live.
-function vatTaxSubtotalCategory(rate: number, vatExempt: boolean): Record<string, unknown> {
+// (see amtCents()'s own comment). BR-K-10/BR-E-10 require either field
+// whenever category is 'K'/'E', satisfied here since this is the one place
+// they're actually allowed to live. §19 UStG has no EU-wide VATEX code (a
+// purely national exemption reason) — BR-E-10 accepts the free-text
+// TaxExemptionReason alone without a TaxExemptionReasonCode, so
+// kleinunternehmer only sets the text, unlike intra-community's VATEX-EU-IC.
+function vatTaxSubtotalCategory(rate: number, vatMode: VatMode): Record<string, unknown> {
   return {
-    ...vatTaxCategory(rate, vatExempt),
-    ...(vatExempt ? { "cbc:TaxExemptionReasonCode": "VATEX-EU-IC", "cbc:TaxExemptionReason": "Innergemeinschaftliche Lieferung" } : {}),
+    ...vatTaxCategory(rate, vatMode),
+    ...(vatMode === "intra-community" ? { "cbc:TaxExemptionReasonCode": "VATEX-EU-IC", "cbc:TaxExemptionReason": "Innergemeinschaftliche Lieferung" } : {}),
+    ...(vatMode === "kleinunternehmer" ? { "cbc:TaxExemptionReason": "Gemäß § 19 UStG wird keine Umsatzsteuer berechnet." } : {}),
   };
 }
 
@@ -254,7 +274,7 @@ function computeRateGroups(lines: ComputedLine[], discountAmount: number, shippi
     .sort((a, b) => b.rate - a.rate);
 }
 
-function taxTotal(rateGroups: RateGroup[], vatExempt: boolean): UblInvoice["cac:TaxTotal"] {
+function taxTotal(rateGroups: RateGroup[], vatMode: VatMode): UblInvoice["cac:TaxTotal"] {
   const totalTaxCents = rateGroups.reduce((sum, g) => sum + g.taxCents, 0);
   return [
     {
@@ -262,13 +282,13 @@ function taxTotal(rateGroups: RateGroup[], vatExempt: boolean): UblInvoice["cac:
       "cac:TaxSubtotal": rateGroups.map((g) => ({
         ...amtCents("cbc:TaxableAmount", g.taxableCents),
         ...amtCents("cbc:TaxAmount", g.taxCents),
-        "cac:TaxCategory": vatTaxSubtotalCategory(g.rate, vatExempt),
+        "cac:TaxCategory": vatTaxSubtotalCategory(g.rate, vatMode),
       })),
     },
   ] as unknown as UblInvoice["cac:TaxTotal"];
 }
 
-function allowanceCharges(rateGroups: RateGroup[], vatExempt: boolean): UblInvoice["cac:AllowanceCharge"] {
+function allowanceCharges(rateGroups: RateGroup[], vatMode: VatMode): UblInvoice["cac:AllowanceCharge"] {
   // Loosely typed here (build as Record<string, unknown>, cast once at the
   // return boundary) — VAT_CATEGORY's literal type widens to plain
   // `string` the moment it's read through an intermediate variable, which
@@ -277,7 +297,7 @@ function allowanceCharges(rateGroups: RateGroup[], vatExempt: boolean): UblInvoi
   // exact same widening issue.
   const entries: Record<string, unknown>[] = [];
   for (const g of rateGroups) {
-    const taxCategory = vatTaxCategory(g.rate, vatExempt);
+    const taxCategory = vatTaxCategory(g.rate, vatMode);
     if (g.allowanceCents > 0) {
       entries.push({
         "cbc:ChargeIndicator": "false",
@@ -298,7 +318,7 @@ function allowanceCharges(rateGroups: RateGroup[], vatExempt: boolean): UblInvoi
   return entries.length ? (entries as unknown as UblInvoice["cac:AllowanceCharge"]) : undefined;
 }
 
-function invoiceLines(lines: ComputedLine[], vatExempt: boolean): UblInvoice["cac:InvoiceLine"] {
+function invoiceLines(lines: ComputedLine[], vatMode: VatMode): UblInvoice["cac:InvoiceLine"] {
   return lines.map(({ item, netCents }, i) => {
     const netUnitPrice = item.unitPrice / (1 + item.taxRatePercent / 100);
     return {
@@ -307,7 +327,7 @@ function invoiceLines(lines: ComputedLine[], vatExempt: boolean): UblInvoice["ca
       ...amtCents("cbc:LineExtensionAmount", netCents),
       "cac:Item": {
         "cbc:Name": item.variantName ? `${item.productName} (${item.variantName})` : item.productName,
-        "cac:ClassifiedTaxCategory": vatTaxCategory(item.taxRatePercent, vatExempt),
+        "cac:ClassifiedTaxCategory": vatTaxCategory(item.taxRatePercent, vatMode),
       },
       // Unit price, not a summed/reconciled total — kept as the plain
       // (unrounded-to-cent) net unit price for reference; BR-CO-10 only
@@ -372,10 +392,10 @@ function paymentTerms(note: string): UblInvoice["cac:PaymentTerms"] {
 // Full original invoice — one line per order item, positive amounts,
 // InvoiceTypeCode 380 ("Commercial invoice").
 export function buildEInvoiceData(order: InvoiceOrder, seller: InvoiceSeller): Invoice {
-  const vatExempt = Boolean(order.vatExempt);
+  const vatMode: VatMode = order.kleinunternehmer ? "kleinunternehmer" : order.vatExempt ? "intra-community" : "standard";
   const lines = computeLines(order.items);
   const rateGroups = computeRateGroups(lines, order.discountAmount, order.shippingCost);
-  const allowanceChargeEntries = allowanceCharges(rateGroups, vatExempt);
+  const allowanceChargeEntries = allowanceCharges(rateGroups, vatMode);
   const paid = isPaidImmediately(order.paymentMethodTitle);
 
   return {
@@ -399,13 +419,13 @@ export function buildEInvoiceData(order: InvoiceOrder, seller: InvoiceSeller): I
       // invoice's BT-115 is 0 (see legalMonetaryTotal()'s `prepaid` param).
       ...(paid ? {} : { "cac:PaymentTerms": paymentTerms("Zahlbar sofort ohne Abzug.") }),
       ...(allowanceChargeEntries ? { "cac:AllowanceCharge": allowanceChargeEntries } : {}),
-      "cac:TaxTotal": taxTotal(rateGroups, vatExempt),
+      "cac:TaxTotal": taxTotal(rateGroups, vatMode),
       // `order.total` is what the customer was actually charged at
       // checkout — any cent of drift against this breakdown's own
       // TaxInclusiveAmount is declared via PayableRoundingAmount inside
       // legalMonetaryTotal(), not silently absorbed into either figure.
       "cac:LegalMonetaryTotal": legalMonetaryTotal(rateGroups, Math.round(order.total * 100), paid),
-      "cac:InvoiceLine": invoiceLines(lines, vatExempt),
+      "cac:InvoiceLine": invoiceLines(lines, vatMode),
     },
   };
 }
@@ -422,7 +442,7 @@ export function buildEInvoiceData(order: InvoiceOrder, seller: InvoiceSeller): I
 // Stornorechnung-vs-Gutschrift policy (full reversal incl. shipping vs.
 // only returned quantities, no shipping, no discount reproration).
 export function buildCorrectionEInvoiceData(kind: CorrectionInvoiceKind, order: CorrectionInvoiceOrder, seller: InvoiceSeller): Invoice {
-  const vatExempt = Boolean(order.vatExempt);
+  const vatMode: VatMode = order.kleinunternehmer ? "kleinunternehmer" : order.vatExempt ? "intra-community" : "standard";
   const effectiveItems =
     kind === "storno"
       ? order.items
@@ -430,7 +450,7 @@ export function buildCorrectionEInvoiceData(kind: CorrectionInvoiceKind, order:
 
   const lines = computeLines(effectiveItems);
   const rateGroups = computeRateGroups(lines, kind === "storno" ? order.discountAmount : 0, kind === "storno" ? order.shippingCost : 0);
-  const allowanceChargeEntries = allowanceCharges(rateGroups, vatExempt);
+  const allowanceChargeEntries = allowanceCharges(rateGroups, vatMode);
   // No separately-stored "charged total" exists for a correction event the
   // way `order.total` does for the original invoice — the reversal amount
   // *is* this breakdown's own TaxInclusiveAmount, so PayableAmount is
@@ -469,9 +489,9 @@ export function buildCorrectionEInvoiceData(kind: CorrectionInvoiceKind, order:
       // branch above) — payment terms text satisfies the rule instead.
       "cac:PaymentTerms": paymentTerms("Der Rechnungsbetrag wird erstattet."),
       ...(allowanceChargeEntries ? { "cac:AllowanceCharge": allowanceChargeEntries } : {}),
-      "cac:TaxTotal": taxTotal(rateGroups, vatExempt),
+      "cac:TaxTotal": taxTotal(rateGroups, vatMode),
       "cac:LegalMonetaryTotal": legalMonetaryTotal(rateGroups, taxInclusiveCents, false),
-      "cac:InvoiceLine": invoiceLines(lines, vatExempt),
+      "cac:InvoiceLine": invoiceLines(lines, vatMode),
     },
   };
 }