[codex] Document Windows libopus setup

[tianmind-studio]

Summary

• Replace the Windows backend setup note that said Opus is already included with Windows 10 1903+.
• Document that opuslib still needs a native libopus DLL on PATH.
• Add an MSYS2 UCRT64 example and a where.exe opus.dll verification step in both the backend README and Mintlify backend setup guide.

Why

On Windows, the Python opuslib package can be installed while its native libopus DLL is still missing. In that state backend imports or Opus encode/decode paths fail with Could not find Opus library. The setup docs should point contributors at the native dependency instead of implying Windows already satisfies it.

Refresh

• Rebased on main at 1a5824403b68ce47c3b0909577cadc1242ba0d3f.
• Current head: 8766d301c59246f4d36c7d8c8383099e3a052e4d.
• Existing approval remains on the PR; review threads are empty.

Validation

• Reviewed public docs diff for repo-boundary leakage; content stays limited to developer-facing setup guidance.
• git diff --check origin/main...HEAD
• scripts/pre-commit

Docs-only change; no backend tests run.

Related: #7928

[kodjima33]

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