🛡️ Sentinel: [HIGH] Fix path traversal in auth file operations

[rschumann]

🚨 Severity: HIGH
💡 Vulnerability: Path traversal vulnerability in UploadAuthFile and DeleteAuthFile management endpoints. The previous code relied on os.PathSeparator to validate file names, which can be bypassed (e.g., passing / on a Windows host or \ in mixed environments).
🎯 Impact: An attacker could potentially upload arbitrary JSON files to other directories or delete arbitrary JSON files on the system if they have access to the management endpoint.
🔧 Fix: Changed the path validation check to strings.ContainsAny(name, "/\\") to robustly prevent directory traversal attempts across all platforms. This matches the existing secure logic in DownloadAuthFile. Added unit tests in auth_files_security_test.go to explicitly verify the path traversal protection.
✅ Verification: Run go test ./internal/api/handlers/management -run "TestUploadAuthFile_PathTraversal|TestDeleteAuthFile_PathTraversal" -v to ensure the unit tests pass and properly reject path traversal payloads.

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PR created automatically by Jules for task 11016025934345615142 started by @rschumann

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