feat(auth): OIDC/Keycloak SSO-Login (additiv) — v1.2.91

SSO per OpenID Connect (Authorization Code + PKCE) zusätzlich zum lokalen Login.
- Regeln: kein Auto-Provisioning (E-Mail muss als User existieren), Rolle aus DB (nie aus Token), lokaler Login+TOTP unangetastet.
- Migration 0040: oidc_settings (Singleton, client_secret_enc via secrets.Box) + users.oidc_subject.
- internal/services/oidc: Settings-Repo (write-only Secret) + lazy go-oidc Client (testbarer Authenticator-Seam).
- internal/handlers/oidc.go: GET/PUT /oidc/settings (admin), GET /auth/oidc/{settings,login,callback}. Flow-State (state/PKCE/nonce) stateless im 5-min signierten HttpOnly-Cookie (SameSite=Lax). email_verified erzwungen, opportunistisches sub-Linking, Session via setSessionCookie+Signer.
- session.SignBlob/VerifyBlob; users.Get/SetOIDCSubject; main.go-Wiring.
- Frontend: App.tsx /auth/me-Bootstrap (für Cookie-Session nach Callback), Login-SSO-Button + sso_error, Settings OIDC-Card, i18n de/en.
- Tests (guarded EG_FWTEST_DSN): Secret-Roundtrip + Callback (Rolle-aus-DB, no_account, disabled, unverified, nonce, state).
Deps: go-oidc/v3, x/oauth2. Scope v1: nur Login (kein SLO/Refresh).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

go.sum

@@ -19,6 +19,8 @@ github.com/corazawaf/coraza/v3 v3.7.0 h1:LIQqu1r+l6e/U/gyiZeykWaNNBY1TzRLz+aaI+Q
 github.com/corazawaf/coraza/v3 v3.7.0/go.mod h1:dOSt5evqC7EstouEv6ghhui01+oVUwp9X1vybWwqTlo=
 github.com/corazawaf/libinjection-go v0.3.2 h1:9rrKt0lpg4WvUXt+lwS06GywfqRXXsa/7JcOw5cQLwI=
 github.com/corazawaf/libinjection-go v0.3.2/go.mod h1:Ik/+w3UmTWH9yn366RgS9D95K3y7Atb5m/H/gXzzPCk=
+github.com/coreos/go-oidc/v3 v3.18.0 h1:V9orjXynvu5wiC9SemFTWnG4F45v403aIcjWo0d41+A=
+github.com/coreos/go-oidc/v3 v3.18.0/go.mod h1:DYCf24+ncYi+XkIH97GY1+dqoRlbaSI26KVTCI9SrY4=
 github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
@@ -190,6 +192,8 @@ golang.org/x/mod v0.35.0 h1:Ww1D637e6Pg+Zb2KrWfHQUnH2dQRLBQyAtpr/haaJeM=
 golang.org/x/mod v0.35.0/go.mod h1:+GwiRhIInF8wPm+4AoT6L0FA1QWAad3OMdTRx4tFYlU=
 golang.org/x/net v0.53.0 h1:d+qAbo5L0orcWAr0a9JweQpjXF19LMXJE8Ey7hwOdUA=
 golang.org/x/net v0.53.0/go.mod h1:JvMuJH7rrdiCfbeHoo3fCQU24Lf5JJwT9W3sJFulfgs=
+golang.org/x/oauth2 v0.36.0 h1:peZ/1z27fi9hUOFCAZaHyrpWG5lwe0RJEEEeH0ThlIs=
+golang.org/x/oauth2 v0.36.0/go.mod h1:YDBUJMTkDnJS+A4BP4eZBjCqtokkg1hODuPjwiGPO7Q=
 golang.org/x/sync v0.20.0 h1:e0PTpb7pjO8GAtTs2dQ6jYa5BWYlMuX047Dco/pItO4=
 golang.org/x/sync v0.20.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=
 golang.org/x/sys v0.44.0 h1:ildZl3J4uzeKP07r2F++Op7E9B29JRUy+a27EibtBTQ=