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/users/UserCreatePage.tsx

@@ -31,6 +31,7 @@ type UserSchemaField = {
   required?: boolean;
   adminOnly?: boolean;
   validation?: string;
+  isLoginId?: boolean;
 };
 
 type UserFormValues = UserCreateRequest & { metadata: Record<string, unknown> };
@@ -65,7 +66,6 @@ function UserCreatePage() {
   } = useForm<UserFormValues>({
     defaultValues: {
       email: "",
-      loginId: "",
       password: "",
       name: "",
       phone: "",
@@ -274,26 +274,6 @@ function UserCreatePage() {
               )}
             </div>
 
-            <div className="space-y-2">
-              <Label htmlFor="loginId">
-                {t("ui.admin.users.create.form.login_id", "로그인 ID (선택)")}
-              </Label>
-              <Input
-                id="loginId"
-                placeholder={t(
-                  "ui.admin.users.create.form.login_id_placeholder",
-                  "사번 또는 아이디",
-                )}
-                {...register("loginId")}
-              />
-              <p className="text-[10px] text-muted-foreground">
-                {t(
-                  "msg.admin.users.create.form.login_id_help",
-                  "이메일/전화번호 외에 별도의 식별자로 로그인할 때 사용합니다.",
-                )}
-              </p>
-            </div>
-
             <div className="space-y-2">
               <div className="flex items-center justify-between">
                 <Label htmlFor="password">
@@ -470,6 +450,11 @@ function UserCreatePage() {
                             Admin Only
                           </span>
                         )}
+                        {field.isLoginId && (
+                          <span className="ml-2 text-[10px] bg-green-500/10 text-green-600 px-1.5 py-0.5 rounded uppercase font-bold">
+                            {t("ui.admin.users.create.form.is_login_id", "로그인 ID")}
+                          </span>
+                        )}
                       </Label>
 
                       <Input