feat: add OAuth 2.0 token endpoint

Add POST /api/v1/oauth/token supporting authorization_code and
refresh_token grant types. Validates PKCE, exchanges codes for
JWT access tokens with refresh token rotation. Uses the shared
RefreshSession helper for the refresh grant.

pkg/modules/auth/oauth2server/token.go

@@ -0,0 +1,144 @@
+// 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 oauth2server
+
+import (
+	"net/http"
+
+	"code.vikunja.io/api/pkg/config"
+	"code.vikunja.io/api/pkg/db"
+	"code.vikunja.io/api/pkg/models"
+	"code.vikunja.io/api/pkg/modules/auth"
+	"code.vikunja.io/api/pkg/user"
+
+	"github.com/labstack/echo/v5"
+)
+
+// TokenResponse is the OAuth 2.0 token response.
+type TokenResponse struct {
+	AccessToken  string `json:"access_token"`
+	TokenType    string `json:"token_type"`
+	ExpiresIn    int64  `json:"expires_in"`
+	RefreshToken string `json:"refresh_token"`
+}
+
+// tokenRequest holds the JSON body of a POST /oauth/token request.
+type tokenRequest struct {
+	GrantType    string `json:"grant_type"`
+	Code         string `json:"code"`
+	ClientID     string `json:"client_id"`
+	RedirectURI  string `json:"redirect_uri"`
+	CodeVerifier string `json:"code_verifier"`
+	RefreshToken string `json:"refresh_token"`
+}
+
+// HandleToken handles POST /oauth/token.
+// Supports grant_type=authorization_code and grant_type=refresh_token.
+func HandleToken(c *echo.Context) error {
+	var req tokenRequest
+	if err := c.Bind(&req); err != nil {
+		return echo.NewHTTPError(http.StatusBadRequest, "Invalid request body")
+	}
+
+	switch req.GrantType {
+	case "authorization_code":
+		return handleAuthorizationCodeGrant(c, &req)
+	case "refresh_token":
+		return handleRefreshTokenGrant(c, &req)
+	default:
+		return &models.ErrOAuthInvalidGrantType{}
+	}
+}
+
+func handleAuthorizationCodeGrant(c *echo.Context, req *tokenRequest) error {
+	s := db.NewSession()
+	defer s.Close()
+
+	// Look up and delete the authorization code (single-use)
+	oauthCode, err := models.GetAndDeleteOAuthCode(s, req.Code)
+	if err != nil {
+		_ = s.Rollback()
+		return err
+	}
+
+	// Validate client_id matches
+	if oauthCode.ClientID != req.ClientID {
+		_ = s.Rollback()
+		return &models.ErrOAuthClientNotFound{}
+	}
+
+	// Validate redirect_uri matches
+	if oauthCode.RedirectURI != req.RedirectURI {
+		_ = s.Rollback()
+		return &models.ErrOAuthInvalidRedirectURI{}
+	}
+
+	// Verify PKCE
+	if !VerifyPKCE(req.CodeVerifier, oauthCode.CodeChallenge, oauthCode.CodeChallengeMethod) {
+		_ = s.Rollback()
+		return &models.ErrOAuthPKCEVerifyFailed{}
+	}
+
+	// Create a session (reuses existing session infrastructure)
+	deviceInfo := c.Request().UserAgent()
+	ipAddress := c.RealIP()
+	session, err := models.CreateSession(s, oauthCode.UserID, deviceInfo, ipAddress, false)
+	if err != nil {
+		_ = s.Rollback()
+		return err
+	}
+
+	u, err := user.GetUserByID(s, oauthCode.UserID)
+	if err != nil {
+		_ = s.Rollback()
+		return err
+	}
+
+	// Generate JWT
+	accessToken, err := auth.NewUserJWTAuthtoken(u, session.ID)
+	if err != nil {
+		_ = s.Rollback()
+		return err
+	}
+
+	if err := s.Commit(); err != nil {
+		return err
+	}
+
+	c.Response().Header().Set("Cache-Control", "no-store")
+	return c.JSON(http.StatusOK, TokenResponse{
+		AccessToken:  accessToken,
+		TokenType:    "bearer",
+		ExpiresIn:    config.ServiceJWTTTLShort.GetInt64(),
+		RefreshToken: session.RefreshToken,
+	})
+}
+
+func handleRefreshTokenGrant(c *echo.Context, req *tokenRequest) error {
+	result, err := auth.RefreshSession(req.RefreshToken)
+	if err != nil {
+		return err
+	}
+
+	c.Response().Header().Set("Cache-Control", "no-store")
+	return c.JSON(http.StatusOK, TokenResponse{
+		AccessToken:  result.AccessToken,
+		TokenType:    "bearer",
+		ExpiresIn:    result.ExpiresIn,
+		RefreshToken: result.NewRefreshToken,
+	})
+}