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

[rschumann]

🚨 Severity: HIGH
💡 Vulnerability: Path traversal checks in file handlers used os.PathSeparator, which allowed attackers to use backslashes (\) to traverse out of restricted directories when running on Linux (where the separator is /). Additionally, UploadAuthFile did not properly validate paths within multipart form data.
🎯 Impact: An attacker could potentially download, delete, or overwrite arbitrary files on the host filesystem by supplying crafted filenames such as ..\..\etc\passwd via the Management API.
🔧 Fix:

• Replaced strings.Contains(name, string(os.PathSeparator)) with explicit strings.ContainsAny(name, "/\\") across all file handlers for true cross-platform protection.
• Reordered execution logic in UploadAuthFile and DeleteAuthFile to perform input validation before the authManager == nil check, allowing inputs to fail fast without requiring full system initialization.
• Added explicit multipart file.Filename validation in UploadAuthFile.
  ✅ Verification: Unit tests within auth_files_security_test.go were updated to run on all platforms, explicitly asserting that backslash traversal attempts are properly rejected with a 400 Bad Request. Tested and passed via go test ./internal/api/handlers/management/ -v -run PathTraversal.

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PR created automatically by Jules for task 13176354521863178677 started by @rschumann

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