docs: fix redirect uri in readme

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Co-Authored-By: Claude Opus 4.5 <[e-mail]>

README.md

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 register an oauth client in the quickslice admin ui at `https://zzstoatzz-quickslice-status.fly.dev/`
 
-redirect uri: `https://status.zzstoatzz.io/callback`
+redirect uri: `https://status.zzstoatzz.io/`
 
 ## lexicons