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/repositories/user_repo.go

@@ -21,6 +21,7 @@ var (
 	ErrTooManyAttempts    = errors.New("too many attempts")
 	ErrRateLimitExceeded  = errors.New("rate limit exceeded")
 	ErrAppleAuthNotFound  = errors.New("apple social auth not found")
+	ErrGoogleAuthNotFound = errors.New("google social auth not found")
 )
 
 // UserRepository handles user-related database operations
@@ -511,3 +512,27 @@ func (r *UserRepository) CreateAppleSocialAuth(auth *models.AppleSocialAuth) err
 func (r *UserRepository) UpdateAppleSocialAuth(auth *models.AppleSocialAuth) error {
 	return r.db.Save(auth).Error
 }
+
+// --- Google Social Auth Methods ---
+
+// FindByGoogleID finds a Google social auth by Google ID
+func (r *UserRepository) FindByGoogleID(googleID string) (*models.GoogleSocialAuth, error) {
+	var auth models.GoogleSocialAuth
+	if err := r.db.Where("google_id = ?", googleID).First(&auth).Error; err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return nil, ErrGoogleAuthNotFound
+		}
+		return nil, err
+	}
+	return &auth, nil
+}
+
+// CreateGoogleSocialAuth creates a new Google social auth record
+func (r *UserRepository) CreateGoogleSocialAuth(auth *models.GoogleSocialAuth) error {
+	return r.db.Create(auth).Error
+}
+
+// UpdateGoogleSocialAuth updates a Google social auth record
+func (r *UserRepository) UpdateGoogleSocialAuth(auth *models.GoogleSocialAuth) error {
+	return r.db.Save(auth).Error
+}