🛡️ Sentinel: [CRITICAL] Fix Path Traversal in File Upload/Delete Handlers

[rschumann]

🚨 Severity: CRITICAL
💡 Vulnerability: UploadAuthFile and DeleteAuthFile checked for path traversal in the user-supplied name query parameter using only os.PathSeparator. On a Linux server, os.PathSeparator is /, meaning Windows-style path traversal payloads (e.g., ..\..\secrets.json) could bypass the validation. When combined with the subsequent filepath.Join(), this could allow attackers to write or delete files outside the intended AuthDir.
🎯 Impact: Remote, unauthenticated attackers (if management endpoints are exposed) could upload arbitrary files to the server's filesystem, or delete critical configuration/auth files, leading to remote code execution (RCE) or denial of service (DoS).
🔧 Fix: Updated the validation to explicitly check for both forward and backward slashes using strings.ContainsAny(name, "/\\"). Moved the check to the very top of the handlers so malicious requests fail immediately (Fast Fail), even before backend authManager checks.
✅ Verification: Unit tests for path traversal have been successfully added to auth_files_security_test.go and verified running go test ./internal/api/handlers/management -run Test.*AuthFile_PathTraversal.

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PR created automatically by Jules for task 6700445860179158313 started by @rschumann

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