🛡️ Sentinel: [CRITICAL] Fix path traversal in auth management handlers

[rschumann]

🚨 Severity: CRITICAL
💡 Vulnerability: The UploadAuthFile and DeleteAuthFile management endpoints in internal/api/handlers/management/auth_files.go were using strings.Contains(name, string(os.PathSeparator)) to validate filename inputs. This is insufficient because it only checks the host OS separator, making the application vulnerable to cross-platform traversal bypassing (e.g. using \ characters on Linux instances).
🎯 Impact: An attacker could bypass path validation checks by using alternate path separators, allowing them to read, write, or delete arbitrary files outside of the authorized AuthDir directory if the operating system or underlying Go path functions eventually normalize the path.
🔧 Fix: Replaced the vulnerable validation check with the Sentinel pattern strings.ContainsAny(name, "/\\"), explicitly denying both forward and backward slashes. Also documented the vulnerability and learning in .jules/sentinel.md.
✅ Verification: Verified by checking code formatting, passing go vet ./internal/api/handlers/management, and successfully running go test ./... -short globally and go test ./internal/api/handlers/management -run TestDownloadAuthFile_PathTraversal specifically.

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PR created automatically by Jules for task 7660994911729275407 started by @rschumann

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