feat: implement multi-identifier architecture (Issue #496)

- Database: Add user_login_ids table for 1:N identifier mapping and remove legacy login_id column
- Kratos: Update identity schema to use custom_login_ids array instead of a single id trait
- Backend: Implement syncCustomLoginIDs to collect isLoginId fields across tenant schemas
- Backend: Add backtracking logic to auto-assign session tenant based on used login identifier
- Backend: Add 409 Conflict exception handling for Create/Update operations
- AdminFront: Refactor UserDetailPage to a tabbed grid layout (Info, Tenants, Security)
- AdminFront: Show '로그인 ID' badge on tenant schema fields used for authentication
- UserFront: Remove legacy optional 'Login ID' input from signup flow
- Tests: Add multi-identifier repository tests and update handler tests

backend/internal/repository/user_repository.go

@@ -21,6 +21,12 @@ type UserRepository interface {
 	CountByTenantIDs(ctx context.Context, tenantIDs []string) (map[string]int64, error)
 	CountByCompanyCodes(ctx context.Context, codes []string) (map[string]int64, error)
 	Delete(ctx context.Context, id string) error
+
+	// Multiple identifiers support
+	UpdateUserLoginIDs(ctx context.Context, userID string, loginIDs []domain.UserLoginID) error
+	GetUserLoginIDs(ctx context.Context, userID string) ([]domain.UserLoginID, error)
+	IsLoginIDTaken(ctx context.Context, loginID string) (bool, error)
+	FindTenantIDByLoginID(ctx context.Context, loginID string) (string, error)
 }
 
 type userRepository struct {
@@ -193,3 +199,45 @@ func (r *userRepository) List(ctx context.Context, offset, limit int, search str
 func (r *userRepository) Delete(ctx context.Context, id string) error {
 	return r.db.WithContext(ctx).Delete(&domain.User{}, "id = ?", id).Error
 }
+
+func (r *userRepository) UpdateUserLoginIDs(ctx context.Context, userID string, loginIDs []domain.UserLoginID) error {
+	return r.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
+		// Delete existing login IDs for this user
+		if err := tx.Where("user_id = ?", userID).Delete(&domain.UserLoginID{}).Error; err != nil {
+			return err
+		}
+
+		// Insert new login IDs if any
+		if len(loginIDs) > 0 {
+			if err := tx.Create(&loginIDs).Error; err != nil {
+				return err
+			}
+		}
+
+		return nil
+	})
+}
+
+func (r *userRepository) GetUserLoginIDs(ctx context.Context, userID string) ([]domain.UserLoginID, error) {
+	var results []domain.UserLoginID
+	if err := r.db.WithContext(ctx).Where("user_id = ?", userID).Find(&results).Error; err != nil {
+		return nil, err
+	}
+	return results, nil
+}
+
+func (r *userRepository) IsLoginIDTaken(ctx context.Context, loginID string) (bool, error) {
+	var count int64
+	if err := r.db.WithContext(ctx).Model(&domain.UserLoginID{}).Where("login_id = ?", loginID).Count(&count).Error; err != nil {
+		return false, err
+	}
+	return count > 0, nil
+}
+
+func (r *userRepository) FindTenantIDByLoginID(ctx context.Context, loginID string) (string, error) {
+	var record domain.UserLoginID
+	if err := r.db.WithContext(ctx).Where("login_id = ?", loginID).First(&record).Error; err != nil {
+		return "", err
+	}
+	return record.TenantID, nil
+}