[codex] Clarify Windows backend setup docs

[tianmind-studio]

Summary

• Update the backend README quick setup to install and verify Python 3.11.
• Align the Mintlify backend setup guide with the backend Docker images' Python 3.11 runtime.
• Replace the outdated Python 3.9-3.12 troubleshooting guidance with Python 3.11.
• Document the Windows gcloud ADC path and use PowerShell Copy-Item syntax in the Windows credentials copy step.

Why

The backend Docker images and backend/AGENTS.md use Python 3.11. The public setup guide still allowed Python 3.9-3.12, which can send Windows contributors into a mismatched local environment before they create their virtualenv. The Windows credential copy example was also under a PowerShell tab but used cmd-style %APPDATA% expansion.

Validation

• git diff --check
• C:\Program Files\Git\bin\bash.exe scripts/pre-commit
• GitHub Actions Lint Check run 5002 -> success

Docs-only change; no backend tests run.

[kodjima33]

Docs (clarify Windows backend setup) — approve only; Windows-specific claims not independently verified.