Persist OIDC config in a single-row oidc_settings table; non-empty DB
fields override the OIDC_* env vars (env = bootstrap default). The
Authlib client is rebuilt when config changes.
- GET/PUT /auth/oidc/settings — admin only, via JWT OR API key. The
API-key path is the recovery channel when OIDC-only mode is on and
OIDC is misconfigured (avoids total lockout).
- client_secret is write-only: never returned (has_client_secret bool),
preserved when the field is left blank on update.
- /auth/config, login/link/callback now use the effective (DB|env)
config so enabling OIDC needs no redeploy.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
