feat(api/v2): add OpenID Connect callback on /api/v2

Port the OIDC callback to Huma, reusing openid.AuthenticateCallback. The
route is only registered when OpenID is enabled; unknown providers still 404
per request. v1's bespoke {message, details} error body is replaced by
standard RFC 9457, folding the provider detail into the structured error.

pkg/routes/api/v2/auth_openid.go

@@ -0,0 +1,93 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package apiv2
+
+import (
+	"context"
+	"errors"
+	"net/http"
+
+	"code.vikunja.io/api/pkg/config"
+	"code.vikunja.io/api/pkg/models"
+	"code.vikunja.io/api/pkg/modules/auth"
+	"code.vikunja.io/api/pkg/modules/auth/openid"
+
+	"github.com/danielgtaylor/huma/v2"
+)
+
+func init() { AddRouteRegistrar(RegisterOpenIDRoutes) }
+
+// RegisterOpenIDRoutes wires the OpenID Connect callback endpoint. It is only
+// registered when OpenID is enabled; individual providers are still resolved per
+// request, so an unknown provider key 404s even when others are configured.
+func RegisterOpenIDRoutes(api huma.API) {
+	if !config.AuthOpenIDEnabled.GetBool() {
+		return
+	}
+
+	Register(api, huma.Operation{
+		OperationID:   "auth-openid-callback",
+		Summary:       "Authenticate with OpenID Connect",
+		Description:   "Exchanges the authorization code returned by an OpenID Connect provider for a Vikunja JWT, creating or updating the matching user. A long-lived refresh token is set as an HttpOnly cookie. When the resolved user has 2FA enabled, the call returns 412 and must be retried with totp_passcode set.",
+		Method:        http.MethodPost,
+		Path:          "/auth/openid/{provider}/callback",
+		DefaultStatus: http.StatusOK,
+		Tags:          []string{"auth"},
+		Security:      publicSecurity,
+	}, authOpenIDCallback)
+}
+
+func authOpenIDCallback(ctx context.Context, in *struct {
+	Provider string          `path:"provider" doc:"The OpenID Connect provider key as returned by the /info endpoint."`
+	Body     openid.Callback `doc:"The provider callback, carrying the authorization code."`
+}) (*authTokenBody, error) {
+	u, err := openid.AuthenticateCallback(ctx, &in.Body, in.Provider) //nolint:contextcheck // resolves providers from a cached, context-less map and runs OIDC discovery on its own background context, like the v1 callback.
+	if err != nil {
+		return nil, translateOpenIDError(err)
+	}
+
+	deviceInfo, ipAddress := requestClientInfo(ctx)
+	// OIDC logins are not "remember me" sessions; v1 always issues a short one.
+	token, err := auth.IssueUserToken(ctx, u, deviceInfo, ipAddress, false)
+	if err != nil {
+		return nil, translateDomainError(err)
+	}
+
+	if ec := echoContextFromCtx(ctx); ec != nil {
+		auth.WriteUserAuthCookies(ec, token)
+	}
+
+	out := &authTokenBody{CacheControl: "no-store"}
+	out.Body.Token = token.AccessToken
+	return out, nil
+}
+
+// translateOpenIDError maps OIDC callback errors to RFC 9457 responses.
+// ErrOpenIDBadRequestWithDetails carries no HTTP semantics of its own (v1 renders
+// it with a bespoke {message, details} body), so v2 maps it to a 400 with the
+// provider detail attached as a structured error detail rather than porting the
+// bespoke shape. Everything else flows through translateDomainError.
+func translateOpenIDError(err error) error {
+	var detailedErr *models.ErrOpenIDBadRequestWithDetails
+	if errors.As(err, &detailedErr) {
+		return huma.Error400BadRequest(detailedErr.Message, &huma.ErrorDetail{
+			Message: "The identity provider rejected the request.",
+			Value:   detailedErr.Details,
+		})
+	}
+	return translateDomainError(err)
+}