🛡️ Sentinel: [HIGH] Fix path traversal in auth handlers

[rschumann]

Severity: HIGH
Vulnerability: Path Traversal
Impact: A Windows-style directory traversal payload (e.g. ..\..\secret.json) could bypass the os.PathSeparator check on non-Windows servers, allowing arbitrary file read/write/delete in the application filesystem.
Fix: Updated the checks in UploadAuthFile and DeleteAuthFile to explicitly check for both forward (/) and backward (\) slashes, matching the existing secure implementation in DownloadAuthFile.
Verification: All relevant management security tests pass, ensuring that encoded and alternate-slash path traversal payloads are correctly rejected.

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PR created automatically by Jules for task 18279690153779998204 started by @rschumann

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