feat(user): support fixed UUID registration and enhance bulk import results

- Added support for fixed UUIDs during bulk registration (Search-first + ExternalID mapping)
- Implemented idempotency and visibility restoration for soft-deleted users
- Enhanced bulk upload UI to show 'New/Updated/Unchanged' status and modified fields
- Added logic to reclaim identifiers (login_id) from colliding records
- Added frontend E2E and backend unit tests for UUID integrity and conflict handling
- Fixed i18n, formatting, and mock tests to satisfy code-check
- Applied 'go fix' for 'omitzero' tags and general Go standards

backend/internal/service/tenant_service_test.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/mock"
+	"gorm.io/gorm"
 )
 
 // --- Local Mocks to avoid collisions ---
@@ -185,6 +186,10 @@ func (m *MockUserRepoForTenant) FindTenantIDByLoginID(ctx context.Context, login
 	return "", nil
 }
 
+func (m *MockUserRepoForTenant) DB() *gorm.DB {
+	return nil
+}
+
 // --- Tests ---
 
 func TestTenantService_RegisterTenant_AutoVerify(t *testing.T) {