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>

internal/database/migrations/0040_oidc_settings.sql

@@ -0,0 +1,35 @@
+-- +goose Up
+-- +goose StatementBegin
+
+-- OIDC / Keycloak SSO — Singleton-Settings (analog forward_proxy_settings).
+-- client_secret_enc: secrets.Box.Seal-Output (AES-256-GCM), NULL = nicht gesetzt.
+-- Rolle kommt bewusst NICHT aus dem Token, daher keine group/role-claim-Spalten.
+CREATE TABLE IF NOT EXISTS oidc_settings (
+    id                INTEGER     PRIMARY KEY DEFAULT 1,
+    enabled           BOOLEAN     NOT NULL DEFAULT false,
+    issuer_url        TEXT        NOT NULL DEFAULT '',
+    client_id         TEXT        NOT NULL DEFAULT '',
+    client_secret_enc BYTEA,
+    scopes            TEXT        NOT NULL DEFAULT 'openid email profile',
+    email_claim       TEXT        NOT NULL DEFAULT 'email',
+    button_label      TEXT        NOT NULL DEFAULT 'Sign in with SSO',
+    created_at        TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at        TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    CONSTRAINT oidc_settings_singleton CHECK (id = 1)
+);
+INSERT INTO oidc_settings (id) VALUES (1) ON CONFLICT DO NOTHING;
+
+-- Opportunistisches Linking: beim ersten SSO-Login wird der OIDC-'sub'
+-- gespeichert; weicht er später ab, wird der Login abgelehnt. Nullable,
+-- kein Backfill (Match-Schlüssel bleibt die verifizierte E-Mail).
+ALTER TABLE users ADD COLUMN IF NOT EXISTS oidc_subject TEXT;
+
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+
+ALTER TABLE users DROP COLUMN IF EXISTS oidc_subject;
+DROP TABLE IF EXISTS oidc_settings;
+
+-- +goose StatementEnd