fix: Robustheits-Luecken aus zweitem, unabhaengigem Sicherheitsaudit behoben

Ein zweiter, unabhaengiger Audit-Durchlauf (frischer Blick, keine
Vorkenntnis meiner eigenen Fixes) bestaetigte alle bisherigen Fixes als
korrekt verdrahtet und fand 3 LOW-severity Robustheitsluecken - keine
davon war ausnutzbar, aber alle sind jetzt behoben:

- HarvestsService.create/ExtractionsService.create pruefen jetzt, dass
  referenzierte colonyIds/harvestIds tatsaechlich existieren, bevor die
  Transaktion laeuft - vorher fuehrte eine unbekannte Id zu einem
  ungefangenen 500 mit rohem Prisma-Fehler statt einer sauberen 400.
- Etiketten-Bildelemente durchlaufen jetzt denselben Magic-Byte-Check
  wie Avatar-/Dokument-Uploads, statt pdfkits Bildparser ungepruefte
  Bytes zu uebergeben.
- Webhook-Secret-Redaktion im Audit-Log nutzt jetzt explizites delete
  statt sich auf Prismas JSON-Serialisierung zu verlassen, die
  undefined-Werte verwirft (fragil, falls sich die Serialisierung mal
  aendert).

Co-Authored-By: Claude Sonnet 5 <[email protected]>

ApiCore-Backend/src/label-templates/label-templates.service.spec.ts

@@ -220,6 +220,75 @@ describe('LabelTemplatesService', () => {
       expect(pdf.toString('latin1')).toContain('/Image');
     });
 
+    it('embeds an image element whose bytes match the declared PNG mime type', async () => {
+      // Minimal valid 1x1 transparent PNG, widely used as a test fixture.
+      const onePixelPng =
+        'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNk+A8AAQUBAScY42YAAAAASUVORK5CYII=';
+      const imageElement: LabelElement = {
+        id: 'el-img',
+        type: 'image',
+        x: 5,
+        y: 5,
+        width: 10,
+        height: 10,
+        src: `data:image/png;base64,${onePixelPng}`,
+      };
+      prisma.labelTemplate.findUnique.mockResolvedValue({
+        ...template,
+        elements: [imageElement],
+      });
+      prisma.bottlingBatch.findUnique.mockResolvedValue({
+        id: 'batch-1',
+        jarSize: 500,
+        jarCount: 1,
+        lotNumber: 'L-001',
+        date: new Date('2026-06-01T00:00:00.000Z'),
+        bestBeforeDate: new Date('2028-06-01T00:00:00.000Z'),
+        honeyBatch: { batchNumber: 'HB-001', honeyType: 'Bluetenhonig' },
+        product: { name: 'Glas 500g' },
+      });
+
+      const pdf = await service.renderLabelsPdf('template-1', 'batch-1');
+
+      // pdfkit always lists /ImageB /ImageC /ImageI in a page's ProcSet
+      // regardless of whether anything is actually embedded, so a plain
+      // "/Image" substring check would pass either way - /Subtype /Image is
+      // the actual embedded-XObject marker.
+      expect(pdf.toString('latin1')).toMatch(/\/Subtype\s*\/Image\b/);
+    });
+
+    it('silently skips an image element whose bytes do not match the declared mime type', async () => {
+      const mislabeled = Buffer.from('not-a-png').toString('base64');
+      const imageElement: LabelElement = {
+        id: 'el-img',
+        type: 'image',
+        x: 5,
+        y: 5,
+        width: 10,
+        height: 10,
+        src: `data:image/png;base64,${mislabeled}`,
+      };
+      prisma.labelTemplate.findUnique.mockResolvedValue({
+        ...template,
+        elements: [imageElement],
+      });
+      prisma.bottlingBatch.findUnique.mockResolvedValue({
+        id: 'batch-1',
+        jarSize: 500,
+        jarCount: 1,
+        lotNumber: 'L-001',
+        date: new Date('2026-06-01T00:00:00.000Z'),
+        bestBeforeDate: new Date('2028-06-01T00:00:00.000Z'),
+        honeyBatch: { batchNumber: 'HB-001', honeyType: 'Bluetenhonig' },
+        product: { name: 'Glas 500g' },
+      });
+
+      const pdf = await service.renderLabelsPdf('template-1', 'batch-1');
+
+      expect(pdf.subarray(0, 4).toString()).toBe('%PDF');
+      expect(pdf.toString('latin1')).not.toMatch(/\/Subtype\s*\/Image\b/);
+    });
+
     function countPages(pdf: Buffer): number {
       const matches = pdf.toString('latin1').match(/\/Type\s*\/Page[^s]/g);
       return matches ? matches.length : 0;

ApiCore-Backend/src/label-templates/label-templates.service.ts

@@ -3,6 +3,7 @@ import PDFDocument from 'pdfkit';
 import * as QRCode from 'qrcode';
 import { PrismaService } from '../prisma/prisma.service';
 import { AuditLogService } from '../audit/audit-log.service';
+import { matchesFileSignature } from '../common/file-signature';
 import { LabelTemplate } from '../../generated/prisma';
 import { CreateLabelTemplateDto } from './dto/create-label-template.dto';
 import { UpdateLabelTemplateDto } from './dto/update-label-template.dto';
@@ -62,7 +63,13 @@ function dataUrlToBuffer(dataUrl: string): Buffer | null {
   if (!match) {
     return null;
   }
-  return Buffer.from(match[2], 'base64');
+  const mimeType = match[1] === 'png' ? 'image/png' : 'image/jpeg';
+  const buffer = Buffer.from(match[2], 'base64');
+  // Same magic-byte check as every other upload path (avatar, documents) -
+  // a template's embedded image is only ever entered by a label-template
+  // editor (label-templates:create/update), but there's no reason to hand
+  // pdfkit's image parser bytes that don't match what they claim to be.
+  return matchesFileSignature(buffer, mimeType) ? buffer : null;
 }
 
 @Injectable()