docs(api): use PKCE for ReadMe docs OAuth

[rectifyer]

Summary

The Re-Authorize / Try It OAuth flow on docs.trakt.tv fails with a generic Authorization Error popup after authorizing (refs #878). ReadMe's default authorization code exchange sends the client credentials via HTTP Basic authentication, which auth.trakt.tv does not accept, so the code-for-token exchange 400s and ReadMe surfaces the generic error.

This adds ReadMe's oauth-options to the root x-readme block with usePkce: true. With PKCE enabled, ReadMe replaces the client secret with a code verifier sent in the request body alongside the client_id, which is the exchange shape auth.trakt.tv supports. It also means developer client secrets never route through ReadMe.

Workaround

Until the docs OAuth flow is fully restored, developers testing from the docs must add https://docs.trakt.tv/oauth2-redirect to their app's Redirect URIs at https://app.trakt.tv/settings/apps/api. That is the callback URL ReadMe sends as the redirect_uri for the docs Try It flow, and it currently must be registered on the app for the authorize step to pass.

Testing

• deno task openapi:generate regenerates the spec with the new oauth-options block
• deno task openapi:validate, deno fmt --check, and deno lint pass

[rectifyer]

@seferturan With https://github.com/trakt/trakt-workers/pull/1426 merged and working, this PR should be good to go as well unless you have any issue with enabling this. It seems to be the only way to allow the API docs to authenticate correctly.