Backend: add autoSignup field to OidcConfig, ClickHouse schema, repository,
and admin controller. Gate OIDC login when auto-signup is disabled and user
is not pre-created (returns 403).
Frontend: add OIDC admin page with full CRUD (save/test/delete), role-gated
Admin nav link parsed from JWT, and matching design system styles.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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