feat(09-05): implement convertCert + wire POST /convert (GREEN)

- Add FileResponse interface and FORMAT_MIME map to service
- Implement convertCert: parses any input format (PEM/DER/PFX/P7B) via
  same logic as parseCert; serializes to pem/der/p7b targetFormat
- DER output uses bytesToHex→Buffer.from(hex,'hex') to avoid utf-8
  corruption (Pitfall 1 / T-09-06)
- P7B output: pkcs7.createSignedData + pem.encode (PEM-wrapped PKCS7)
- Wrap all forge ops in try/catch → BadRequestException (T-09-01)
- Controller: add @Body('pemText') + reject when neither file nor pemText
- Fix: re-add NotImplementedException import for mergeCerts stub
- All 23 API cert-manager tests green (including 4 new convertCert)

apps/api/src/cert-manager/cert-manager.service.spec.ts

@@ -397,7 +397,8 @@ describe('convertCert', () => {
 
     // Decode base64 P7B (PEM-wrapped PKCS7) and verify at least 1 cert enclosed
     const p7bContent = Buffer.from(result.content as string, 'base64').toString('utf-8');
-    const p7 = forge.pkcs7.messageFromPem(p7bContent);
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const p7 = forge.pkcs7.messageFromPem(p7bContent) as any;
     expect((p7.certificates as forge.pki.Certificate[]).length).toBeGreaterThanOrEqual(1);
   });