token expire 時要回傳 401, 不是偷偷變成 guest

[chhsiao1981]

Describe the bug

現在機制是 token expire 時會偷偷變成 guest～
但是如此會導致 user / frontend 的設定 (原本的 user)
和 backend (guest) 不一樣～

需要改成直接 return 401～

To Reproduce

Expected behavior

Screenshots

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Additional context

https://github.com/Ptt-official-app/go-openbbsmiddleware/blob/main/api/api_login_required.go#L44