Fix OAuth Callback Error and Improve Binding Robustness

[google-labs-jules]

Wrap the GitHub OAuth callback logic in a try-catch block to return proper JSON error responses instead of raw script errors. Implement robust user binding logic to handle collisions and ensure 1:1 GitHub-to-User mapping, including recovery of orphaned bindings.

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PR created automatically by Jules for task 96846465985348272 started by @OsakaLOOP

[google-labs-jules]

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