Add GET /oauth/{provider}/callback (code+state from query, client_id
and redirect_uri recovered from the state record) alongside the
existing POST callback. Callback now fetches real userinfo
(GitHub api.github.com/user, Google openidconnect userinfo) and
issues a usable local API key on success: new users get the
create_user key, existing users rotate (old key dead, audit
oauth_login + api_key_rotated). Key plaintext appears only once
on the success page/response; failures are 400 HTML/JSON without
secrets, and a missing client_secret fails before any network call.
State remains one-time, 10 minutes, provider/redirect_uri matched.
Tests: 10 new OAuth real-login tests pass; host 27, cloud 249
(4 skipped are the pre-existing real-Postgres tests).
Real end-to-end still needs a registered OAuth App and a public
HTTPS callback entry (server still binds localhost only).
host