Add Google OAuth authentication support

- Add Google OAuth token verification and user lookup/creation
- Add GoogleAuthRequest and GoogleAuthResponse DTOs
- Add GoogleLogin handler in auth_handler.go
- Add google_auth.go service for token verification
- Add FindByGoogleID repository method for user lookup
- Add GoogleID field to User model
- Add Google OAuth configuration (client ID, enabled flag)
- Add i18n translations for Google auth error messages
- Add Google verification email template support

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/handlers/auth_handler.go

@@ -16,10 +16,11 @@ import (
 
 // AuthHandler handles authentication endpoints
 type AuthHandler struct {
-	authService      *services.AuthService
-	emailService     *services.EmailService
-	cache            *services.CacheService
-	appleAuthService *services.AppleAuthService
+	authService       *services.AuthService
+	emailService      *services.EmailService
+	cache             *services.CacheService
+	appleAuthService  *services.AppleAuthService
+	googleAuthService *services.GoogleAuthService
 }
 
 // NewAuthHandler creates a new auth handler
@@ -36,6 +37,11 @@ func (h *AuthHandler) SetAppleAuthService(appleAuth *services.AppleAuthService)
 	h.appleAuthService = appleAuth
 }
 
+// SetGoogleAuthService sets the Google auth service (called after initialization)
+func (h *AuthHandler) SetGoogleAuthService(googleAuth *services.GoogleAuthService) {
+	h.googleAuthService = googleAuth
+}
+
 // Login handles POST /api/auth/login/
 func (h *AuthHandler) Login(c *gin.Context) {
 	var req requests.LoginRequest
@@ -427,3 +433,52 @@ func (h *AuthHandler) AppleSignIn(c *gin.Context) {
 
 	c.JSON(http.StatusOK, response)
 }
+
+// GoogleSignIn handles POST /api/auth/google-sign-in/
+func (h *AuthHandler) GoogleSignIn(c *gin.Context) {
+	var req requests.GoogleSignInRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, responses.ErrorResponse{
+			Error: i18n.LocalizedMessage(c, "error.invalid_request_body"),
+			Details: map[string]string{
+				"validation": err.Error(),
+			},
+		})
+		return
+	}
+
+	if h.googleAuthService == nil {
+		log.Error().Msg("Google auth service not configured")
+		c.JSON(http.StatusInternalServerError, responses.ErrorResponse{
+			Error: i18n.LocalizedMessage(c, "error.google_signin_not_configured"),
+		})
+		return
+	}
+
+	response, err := h.authService.GoogleSignIn(c.Request.Context(), h.googleAuthService, &req)
+	if err != nil {
+		status := http.StatusUnauthorized
+		message := i18n.LocalizedMessage(c, "error.google_signin_failed")
+
+		if errors.Is(err, services.ErrUserInactive) {
+			message = i18n.LocalizedMessage(c, "error.account_inactive")
+		} else if errors.Is(err, services.ErrGoogleSignInFailed) {
+			message = i18n.LocalizedMessage(c, "error.invalid_google_token")
+		}
+
+		log.Debug().Err(err).Msg("Google Sign In failed")
+		c.JSON(status, responses.ErrorResponse{Error: message})
+		return
+	}
+
+	// Send welcome email for new users (async)
+	if response.IsNewUser && h.emailService != nil && response.User.Email != "" {
+		go func() {
+			if err := h.emailService.SendGoogleWelcomeEmail(response.User.Email, response.User.FirstName); err != nil {
+				log.Error().Err(err).Str("email", response.User.Email).Msg("Failed to send Google welcome email")
+			}
+		}()
+	}
+
+	c.JSON(http.StatusOK, response)
+}