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

adminfront/src/features/tenants/routes/TenantSchemaPage.tsx

@@ -97,7 +97,6 @@ export function TenantSchemaPage() {
 
   useEffect(() => {
     const rawSchema = tenantQuery.data?.config?.userSchema;
-    const loginIdField = tenantQuery.data?.config?.loginIdField;
 
     if (Array.isArray(rawSchema)) {
       setFields(
@@ -118,7 +117,7 @@ export function TenantSchemaPage() {
           validation:
             typeof field?.validation === "string" ? field.validation : "",
           unsigned: Boolean(field?.unsigned),
-          isLoginId: field?.key === loginIdField,
+          isLoginId: Boolean(field?.isLoginId),
         })),
       );
     }
@@ -126,13 +125,13 @@ export function TenantSchemaPage() {
 
   const updateMutation = useMutation({
     mutationFn: (newFields: SchemaField[]) => {
-      const loginIdField = newFields.find((f) => f.isLoginId)?.key || "";
+      // Remove legacy loginIdField, keep isLoginId natively in userSchema
+      const newConfig = { ...tenantQuery.data?.config };
+      delete newConfig.loginIdField;
+      newConfig.userSchema = newFields;
+
       return updateTenant(tenantId, {
-        config: {
-          ...tenantQuery.data?.config,
-          userSchema: newFields,
-          loginIdField: loginIdField,
-        },
+        config: newConfig,
       });
     },
     onSuccess: () => {
@@ -344,14 +343,10 @@ export function TenantSchemaPage() {
                   <label className="flex items-center gap-2 cursor-pointer">
                     <input
                       type="checkbox"
-                      checked={field.isLoginId}
-                      onChange={(e) => {
-                        const newFields = fields.map((f, i) => ({
-                          ...f,
-                          isLoginId: i === index ? e.target.checked : false,
-                        }));
-                        setFields(newFields);
-                      }}
+                      checked={field.isLoginId || false}
+                      onChange={(e) =>
+                        updateField(index, { isLoginId: e.target.checked })
+                      }
                       className="w-4 h-4 rounded border-gray-300 text-primary focus:ring-primary"
                     />
                     <span className="text-sm font-medium text-blue-600">