🛡️ Sentinel: [CRITICAL] Fix path traversal vulnerability in auth file management

[rschumann]

🚨 Severity: CRITICAL
💡 Vulnerability: Path Traversal in UploadAuthFile and DeleteAuthFile.
The previous validation used strings.Contains(name, string(os.PathSeparator)) which only checks for the separator of the host OS. This allowed attackers to use the alternative separator (e.g., \ on Linux or / on Windows) to bypass the check, potentially leading to arbitrary file write or deletion (especially on Windows where both separators are valid).

🎯 Impact: An attacker could overwrite or delete critical system files if the server is running on Windows, or create files outside the intended directory.

🔧 Fix: Updated the validation to use strings.ContainsAny(name, "/\\") which explicitly rejects both forward and backward slashes on all platforms.

✅ Verification: Added a new regression test TestAuthFiles_PathTraversal in internal/api/handlers/management/auth_files_security_test.go that verifies both separator types are rejected.

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PR created automatically by Jules for task 12121560632116789410 started by @rschumann

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