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/tenant_handler_test.go

@@ -131,6 +131,22 @@ func (m *MockUserRepoForHandler) CountByCompanyCodes(ctx context.Context, codes
 	return args.Get(0).(map[string]int64), args.Error(1)
 }
 
+func (m *MockUserRepoForHandler) UpdateUserLoginIDs(ctx context.Context, userID string, loginIDs []domain.UserLoginID) error {
+	return nil
+}
+
+func (m *MockUserRepoForHandler) GetUserLoginIDs(ctx context.Context, userID string) ([]domain.UserLoginID, error) {
+	return nil, nil
+}
+
+func (m *MockUserRepoForHandler) IsLoginIDTaken(ctx context.Context, loginID string) (bool, error) {
+	return false, nil
+}
+
+func (m *MockUserRepoForHandler) FindTenantIDByLoginID(ctx context.Context, loginID string) (string, error) {
+	return "", nil
+}
+
 func TestTenantHandler_CreateTenant(t *testing.T) {
 	app := fiber.New()
 	mockSvc := new(MockTenantService)