21st.dev auth-gated registry: document API-key path + quarterly mirror re-verification

[AnayDhawan]

What

Two follow-ups from the v1.0.0 curation:

1. Document the authenticated path for 21st.dev registry installs (account/API key) in references/live-fetch.md, alongside the existing unauthenticated fallbacks (author mirror registries, page fetch).
2. Add the kokonutui.com mirror URLs used by the 21st.dev entries to the scheduled health check scope (Scheduled soft-fail registry health check #3) so a mirror rename/removal surfaces automatically instead of silently breaking three entries.

Context

21st.dev's /r/ endpoint returns {"error":"Authentication required"} unauthenticated (verified 2026-07-16). Current entries route around it; that routing needs docs + rot monitoring.