feat: Introduce Undefined function for Nullable type and refactor DTOs to use Nullable directly

backend/cmd/modeltest/main.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/oklog/ulid/v2"
 	"github.com/timetracker/backend/internal/models"
+	"github.com/timetracker/backend/internal/types"
 )
 
 func main() {
@@ -101,7 +102,7 @@ func testUserModel(ctx context.Context) {
 		Email:      "test@example.com",
 		Password:   "Test@123456",
 		Role:       models.RoleUser,
-		CompanyID:  companyID,
+		CompanyID:  &companyID,
 		HourlyRate: 50.0,
 	}
 
@@ -196,7 +197,7 @@ func testRelationships(ctx context.Context) {
 
 	// Test invalid ID
 	invalidID := ulid.MustNew(ulid.Timestamp(time.Now()), ulid.DefaultEntropy())
-	invalidUser, err := models.GetUserByID(ctx, models.FromULID(invalidID))
+	invalidUser, err := models.GetUserByID(ctx, types.FromULID(invalidID))
 	if err != nil {
 		log.Fatalf("Error getting user with invalid ID: %v", err)
 	}