feat: add buildPdf for cashbox export

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

myteamwallet_backend/src/cashbox-export/cashbox-export.utils.spec.ts

@@ -1,4 +1,4 @@
-import { buildCsv, buildRows } from './cashbox-export.utils';
+import { buildCsv, buildPdf, buildRows } from './cashbox-export.utils';
 
 describe('buildRows', () => {
   const team = (overrides: Partial<{ transactions: any[]; players: any[] }> = {}) => ({
@@ -167,3 +167,26 @@ describe('buildCsv', () => {
     );
   });
 });
+
+describe('buildPdf', () => {
+  it('produces a non-empty valid PDF buffer', async () => {
+    const buffer = await buildPdf(
+      { name: 'Team A' } as any,
+      [
+        { date: '2026-08-05T00:00:00.000Z', type: 'payment', who: 'Alex Muster', note: 'Bar bezahlt', amount: 10, runningTotal: 10 },
+      ],
+      '2026-08-01',
+      '2026-08-31',
+    );
+
+    expect(Buffer.isBuffer(buffer)).toBe(true);
+    expect(buffer.length).toBeGreaterThan(100);
+    expect(buffer.subarray(0, 5).toString('utf-8')).toBe('%PDF-');
+  });
+
+  it('still produces a valid PDF when there are no rows', async () => {
+    const buffer = await buildPdf({ name: 'Team A' } as any, [], '2026-08-01', '2026-08-31');
+
+    expect(buffer.subarray(0, 5).toString('utf-8')).toBe('%PDF-');
+  });
+});