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

[rschumann]

🚨 Severity: HIGH
💡 Vulnerability: The UploadAuthFile and DeleteAuthFile handlers within the management API did not validate the user-provided name query parameter or multipart file.Filename for path traversal sequences (/ or \) until after they performed a nil check on h.authManager. This could leak application state or allow path traversal depending on the initialization timing. Even if checked later, the file upload mechanism was susceptible to path traversal via multipart form filenames.
🎯 Impact: An attacker could potentially write files outside of the configured AuthDir (via upload) or delete arbitrary .json files on the host filesystem (via delete) if they manage to hit the endpoint at the right time, or bypass the query param checks by using the multipart filename attribute.
🔧 Fix:

1. Moved the strings.ContainsAny(name, "/\\") path traversal check for the name parameter to the very top of the UploadAuthFile and DeleteAuthFile handlers, guaranteeing early failure (400 Bad Request) before any state checks or application logic executes.
2. Added a strings.ContainsAny(file.Filename, "/\\") check specifically for the multipart upload form file.Filename to prevent path traversal during file saves.
   ✅ Verification: Expanded TestDownloadAuthFile_PathTraversal in auth_files_security_test.go to explicitly verify that path traversal attempts on POST /upload and DELETE /delete return 400 Bad Request. Ran go test ./internal/api/handlers/management -run TestDownloadAuthFile_PathTraversal to confirm the fix.

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PR created automatically by Jules for task 3727276408922104936 started by @rschumann

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