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/invoicePdf.tsx

@@ -156,6 +156,16 @@ export type InvoiceOrder = {
   // figures) — this flag only controls the exemption note shown here,
   // it doesn't itself change any arithmetic in this file.
   vatExempt?: boolean;
+  // Kleinunternehmerregelung (§19 UStG) — snapshotted per order at checkout
+  // time (frontend's api/checkout/route.ts, from the seller's own
+  // company-settings.kleinunternehmer), never read live off the seller at
+  // render time: an invoice already issued must keep showing the tax
+  // treatment that actually applied at the moment of sale, even if the
+  // tenant later toggles this setting on/off. Takes precedence over
+  // vatExempt below when both would otherwise apply — a Kleinunternehmer
+  // never charges VAT in the first place, so there is nothing left to
+  // "exempt" via the separate intra-community rule.
+  kleinunternehmer?: boolean;
   deliveryMethod: "address" | "packstation";
   street?: string | null;
   packstationNumber?: string | null;
@@ -394,7 +404,9 @@ export function InvoiceDocument({ order, seller }: { order: InvoiceOrder; seller
                     made it read as if Netto were what contained the tax,
                     which is backwards (Netto is the figure with tax
                     already taken out). */}
-                {order.vatExempt ? (
+                {order.kleinunternehmer ? (
+                  <Text style={styles.vatNoteLabel}>Gemäß § 19 UStG wird keine Umsatzsteuer berechnet.</Text>
+                ) : order.vatExempt ? (
                   <Text style={styles.vatNoteLabel}>Steuerfreie innergemeinschaftliche Lieferung (§4 Nr. 1b UStG)</Text>
                 ) : (
                   rateGroups.map((g) => (