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/ory_service.go

@@ -43,7 +43,7 @@ func (o *OryProvider) Name() string {
 func (o *OryProvider) GetMetadata() (*domain.IDPMetadata, error) {
 	return &domain.IDPMetadata{
 		SupportedFields: []string{
-			"id", "login_id", "email", "name", "phone_number",
+			"id", "custom_login_ids", "login_id", "email", "name", "phone_number",
 			"grade", "department", "affiliationType", "companyCode",
 		},
 	}, nil
@@ -67,6 +67,21 @@ func (o *OryProvider) CreateUser(user *domain.BrokerUser, password string) (stri
 		return "", fmt.Errorf("ory provider: identity already exists for email=%s", user.Email)
 	}
 
+	// [New] Check all custom login IDs for collisions
+	for _, lid := range user.CustomLoginIDs {
+		if lid == "" {
+			continue
+		}
+		existing, err := o.findIdentityID(lid)
+		if err != nil {
+			return "", fmt.Errorf("ory provider: search identity failed for %s: %w", lid, err)
+		}
+		if existing != "" {
+			return "", fmt.Errorf("ory provider: identifier %s already exists", lid)
+		}
+	}
+
+	// [Legacy] check single LoginID
 	if user.LoginID != "" {
 		existingLoginID, err := o.findIdentityID(user.LoginID)
 		if err != nil {
@@ -91,13 +106,20 @@ func (o *OryProvider) CreateUser(user *domain.BrokerUser, password string) (stri
 		"email": user.Email,
 		"name":  user.Name,
 	}
-	if user.LoginID != "" {
-		traits["id"] = user.LoginID
+	if len(user.CustomLoginIDs) > 0 {
+		traits["custom_login_ids"] = user.CustomLoginIDs
+	} else if user.LoginID != "" {
+		traits["custom_login_ids"] = []string{user.LoginID}
 	}
+
 	if user.PhoneNumber != "" {
 		traits["phone_number"] = user.PhoneNumber
 	}
 	for k, v := range user.Attributes {
+		// [SoT Fix] Don't let attributes overwrite core traits or use old 'id' trait
+		if k == "id" || k == "email" || k == "custom_login_ids" {
+			continue
+		}
 		traits[k] = v
 	}