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

[rschumann]

🚨 Severity: HIGH
💡 Vulnerability: Path traversal vulnerability in UploadAuthFile and DeleteAuthFile where the name parameter or file.Filename was validated using only the OS path separator (e.g., os.PathSeparator), which allows cross-platform path traversal if a client sends a Windows-style separator (\) to a Linux server, or vice versa.
🎯 Impact: An attacker could potentially write or delete arbitrary files outside the configured AuthDir directory if the application is running in an environment where the differing path separator is interpreted as a directory traversal payload by the filesystem or downstream processes.
🔧 Fix: Replaced the OS-specific validation strings.Contains(name, string(os.PathSeparator)) with cross-platform validation strings.ContainsAny(name, "/\\") for both uploaded file names and the name query parameter in the UploadAuthFile and DeleteAuthFile handlers. Also explicitly validate the file.Filename parsed from the form data before evaluating filepath.Base(). Added missing unit tests for UploadAuthFile and DeleteAuthFile security paths in auth_files_security_test.go.
✅ Verification: Verified by running go test ./internal/api/handlers/management -run "TestUploadAuthFile_PathTraversal|TestDeleteAuthFile_PathTraversal" successfully.

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PR created automatically by Jules for task 15061360802444008909 started by @rschumann

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