[codex] auto-create local dev user

[teddy8023mars]

Summary

• Auto-create and use a local-dev user in non-production local development when OAuth is not configured.
• This keeps local-only workflows from requiring a login for protected local features such as judge history/recent submissions.
• Add context tests for the local-dev path and OAuth-configured opt-out.

Validation

• pnpm check
• pnpm test
• Browser smoke test: http://127.0.0.1:3000/problems shows 544 local problems without login.