feat(auth): add enforceTOTPIfRequired helper for OIDC flow

Extracts a TOTP gate that the OIDC callback will use to enforce 2FA
for users with TOTP enabled. Mirrors the local-login TOTP flow in
pkg/routes/api/v1/login.go. Not yet wired into HandleCallback.

Refs GHSA-8jvc-mcx6-r4cg

pkg/modules/auth/openid/openid.go

@@ -32,6 +32,7 @@ import (
 	"code.vikunja.io/api/pkg/modules/auth"
 	"code.vikunja.io/api/pkg/modules/avatar"
 	"code.vikunja.io/api/pkg/modules/avatar/upload"
+	"code.vikunja.io/api/pkg/modules/keyvalue"
 	"code.vikunja.io/api/pkg/user"
 	"code.vikunja.io/api/pkg/utils"
 
@@ -119,6 +120,38 @@ func (p *Provider) Issuer() (issuerURL string, err error) {
 	return iss.Issuer, nil
 }
 
+// enforceTOTPIfRequired mirrors the TOTP gate from pkg/routes/api/v1/login.go
+// for the OIDC flow. Returns nil when the user does not have TOTP enabled.
+// See GHSA-8jvc-mcx6-r4cg.
+func enforceTOTPIfRequired(s *xorm.Session, u *user.User, totpPasscode string) error {
+	totpEnabled, err := user.TOTPEnabledForUser(s, u)
+	if err != nil {
+		return err
+	}
+	if !totpEnabled {
+		return nil
+	}
+
+	if totpPasscode == "" {
+		return user.ErrInvalidTOTPPasscode{}
+	}
+
+	_, err = user.ValidateTOTPPasscode(s, &user.TOTPPasscode{
+		User:     u,
+		Passcode: totpPasscode,
+	})
+	if err != nil {
+		return err
+	}
+
+	// Reset the counter so old failed attempts don't trip a later lockout.
+	if err := keyvalue.Del(u.GetFailedTOTPAttemptsKey()); err != nil {
+		return err
+	}
+
+	return nil
+}
+
 // HandleCallback handles the auth request callback after redirecting from the provider with an auth code
 // @Summary Authenticate a user with OpenID Connect
 // @Description After a redirect from the OpenID Connect provider to the frontend has been made with the authentication `code`, this endpoint can be used to obtain a jwt token for that user and thus log them in.