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/service/descope_service.go

@@ -97,10 +97,13 @@ func (d *DescopeProvider) CreateUser(user *domain.BrokerUser, password string) (
 		return "", fmt.Errorf("descope provider: user already exists")
 	}
 
-	descopeUser := &descope.UserRequest{}
-	descopeUser.Email = user.Email
-	descopeUser.Phone = normalizedPhone
-	descopeUser.Name = user.Name
+	descopeUser := &descope.UserRequest{
+		User: descope.User{
+			Email: user.Email,
+			Name:  user.Name,
+			Phone: normalizedPhone,
+		},
+	}
 	descopeUser.CustomAttributes = map[string]any{}
 	for k, v := range user.Attributes {
 		descopeUser.CustomAttributes[k] = v