fix: correct Feishu OAuth env var names to match code

[Ansh1372]

Fixes open-webui/open-webui#24094

FEISHU_CLIENT_SCOPE → FEISHU_OAUTH_SCOPE
FEISHU_CLIENT_REDIRECT_URI → FEISHU_REDIRECT_URI

Updated docs to match what the code actually reads.

[pr-validator-bot]

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