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/models/oidc_settings.go

@@ -0,0 +1,21 @@
+package models
+
+import "time"
+
+// OIDCSettings ist die Singleton-Konfiguration für OIDC/Keycloak-SSO.
+// ClientSecretEnc trägt den verschlüsselten Client-Secret (secrets.Box)
+// und wird NIE serialisiert (json:"-").
+type OIDCSettings struct {
+	ID              int       `gorm:"column:id;primaryKey"        json:"id"`
+	Enabled         bool      `gorm:"column:enabled"              json:"enabled"`
+	IssuerURL       string    `gorm:"column:issuer_url"           json:"issuer_url"`
+	ClientID        string    `gorm:"column:client_id"            json:"client_id"`
+	ClientSecretEnc []byte    `gorm:"column:client_secret_enc"    json:"-"`
+	Scopes          string    `gorm:"column:scopes"               json:"scopes"`
+	EmailClaim      string    `gorm:"column:email_claim"          json:"email_claim"`
+	ButtonLabel     string    `gorm:"column:button_label"         json:"button_label"`
+	CreatedAt       time.Time `gorm:"column:created_at"           json:"created_at"`
+	UpdatedAt       time.Time `gorm:"column:updated_at"           json:"updated_at"`
+}
+
+func (OIDCSettings) TableName() string { return "oidc_settings" }