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/handler/auth_handler_async_test.go

@@ -80,7 +80,13 @@ func (m *AsyncMockUserRepo) Create(ctx context.Context, user *domain.User) error
 	}
 	return args.Error(0)
 }
-func (m *AsyncMockUserRepo) Update(ctx context.Context, user *domain.User) error { return nil }
+func (m *AsyncMockUserRepo) Update(ctx context.Context, user *domain.User) error {
+	args := m.Called(ctx, user)
+	if m.createCalled != nil {
+		m.createCalled <- true
+	}
+	return args.Error(0)
+}
 func (m *AsyncMockUserRepo) Delete(ctx context.Context, id string) error         { return nil }
 func (m *AsyncMockUserRepo) FindByEmail(ctx context.Context, email string) (*domain.User, error) {
 	return nil, nil
@@ -114,6 +120,22 @@ func (m *AsyncMockUserRepo) CountByCompanyCodes(ctx context.Context, codes []str
 	return nil, nil
 }
 
+func (m *AsyncMockUserRepo) UpdateUserLoginIDs(ctx context.Context, userID string, loginIDs []domain.UserLoginID) error {
+	return nil
+}
+
+func (m *AsyncMockUserRepo) GetUserLoginIDs(ctx context.Context, userID string) ([]domain.UserLoginID, error) {
+	return nil, nil
+}
+
+func (m *AsyncMockUserRepo) IsLoginIDTaken(ctx context.Context, loginID string) (bool, error) {
+	return false, nil
+}
+
+func (m *AsyncMockUserRepo) FindTenantIDByLoginID(ctx context.Context, loginID string) (string, error) {
+	return "", nil
+}
+
 type AsyncMockRedisRepo struct {
 	mock.Mock
 }
@@ -254,7 +276,7 @@ func TestSignup_AsyncDB_Isolation(t *testing.T) {
 
 		// UserRepo Mocks (Async & Failure)
 		mockUserRepo.createCalled = make(chan bool, 1)
-		mockUserRepo.On("Create", mock.Anything, mock.MatchedBy(func(u *domain.User) bool {
+		mockUserRepo.On("Update", mock.Anything, mock.MatchedBy(func(u *domain.User) bool {
 			return u.Email == email
 		})).Return(errors.New("db connection error"))