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

[rschumann]

🚨 Severity: HIGH
💡 Vulnerability: Weak path traversal checks in the auth_files.go gateway handling (UploadAuthFile, DeleteAuthFile) relied on os.PathSeparator, leaving the server vulnerable to traversal payloads containing alternate separators like \ if maliciously crafted by clients. Additionally, file.Filename lacked explicit validation before filepath.Base().
🎯 Impact: Potential arbitrary file overwrite or deletion on the host filesystem when attackers craft paths designed to bypass simple OS-specific checks.
🔧 Fix: Always use strings.ContainsAny(name, "/\\") whenever extracting a raw filename string from user HTTP requests prior to writing or deleting localized filesystem data.
✅ Verification: Verified via test suite run (go test ./... -short) and specific target checks (go test ./internal/api/handlers/management -run TestDownloadAuthFile_PathTraversal).

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PR created automatically by Jules for task 691622411215999274 started by @rschumann

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