Expose desktop credential identity in status

[Enksodsoon]

Desktop pairing currently returns an authenticated credential status without enough identity for Forge to scope the connection. This additive change returns the authorized default organization ID, authenticated user ID, and opaque credential ID from the existing credential-status route. It continues to omit the bearer token.

The pairing regression checks each identity value against persisted state and asserts that no access token is returned.

Validation on e0cbd0e46bf0000fab89c77b760a1c73b902cd1a:

• Focused pairing API regression and Ruff passed locally.
• GitHub CI passed backend (including the full backend suite with native image runtime), Postgres, web, browser, disposable real-imaging full-stack, containers, repository/dependency security, and CodeQL checks. All required checks passed.
• The local Windows full suite had one OME fallback failure because libvips-42.dll is not installed on this host; the CI backend job installs the native runtime and passed.
• The separate TestSprite Pre-Check reports “No tests detected”; it is not a required check, and the backend/browser/full-stack suites passed.

No existing Viewer account, Forge credential, or live slide data was changed. This PR remains a draft pending review; merge and deployment are still needed before live identity or delivery qualification.