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/UserListPage.tsx

@@ -430,9 +430,6 @@ function UserListPage() {
                         "NAME / EMAIL",
                       )}
                     </TableHead>
-                    <TableHead>
-                      {t("ui.admin.users.list.table.login_id", "LOGIN ID")}
-                    </TableHead>
                     <TableHead>
                       {t("ui.admin.users.list.table.role", "ROLE")}
                     </TableHead>
@@ -514,11 +511,6 @@ function UserListPage() {
                           </div>
                         </div>
                       </TableCell>
-                      <TableCell>
-                        <span className="text-sm font-mono">
-                          {user.loginId || "-"}
-                        </span>
-                      </TableCell>
                       <TableCell>
                         <Badge variant="outline">
                           {t(`ui.admin.role.${user.role}`, user.role)}