fix(pieces) : empêche EntityNotFoundException sur Piece orpheline + UX prévention delete

- Migration FK CASCADE/SET NULL pour toutes les FK vers pieces.id (miroir
  de la fix Composant) + cleanup des orphelins existants avec audit log
- Helper ensurePieceExists() qui catch EntityNotFoundException dans
  MachineStructureController et CustomFieldValueController
- Script SQL standalone scripts/cleanup_orphan_piece_refs.sql pour
  nettoyer la prod sans attendre la migration
- Affiche les machines (avec leur site) utilisant la pièce avant la
  confirmation de suppression

src/Controller/CustomFieldValueController.php

@@ -6,6 +6,7 @@ namespace App\Controller;
 
 use App\Entity\CustomField;
 use App\Entity\CustomFieldValue;
+use App\Entity\Piece;
 use App\Repository\ComposantRepository;
 use App\Repository\CustomFieldRepository;
 use App\Repository\CustomFieldValueRepository;
@@ -15,6 +16,7 @@ use App\Repository\MachineRepository;
 use App\Repository\PieceRepository;
 use App\Repository\ProductRepository;
 use Doctrine\ORM\EntityManagerInterface;
+use Doctrine\ORM\EntityNotFoundException;
 use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
 use Symfony\Component\HttpFoundation\JsonResponse;
 use Symfony\Component\HttpFoundation\Request;
@@ -288,12 +290,31 @@ class CustomFieldValueController extends AbstractController
 
             case 'machinePieceLink':
                 $value->setMachinePieceLink($entity);
-                $value->setPiece($entity->getPiece());
+                $value->setPiece($this->ensurePieceExists($entity->getPiece()));
 
                 break;
         }
     }
 
+    /**
+     * Returns the Piece if its underlying row still exists in DB, otherwise null.
+     * Forces a lazy proxy to initialize via getId() and swallows EntityNotFoundException
+     * so an orphan link to a deleted piece doesn't crash custom-field value writes.
+     */
+    private function ensurePieceExists(?Piece $piece): ?Piece
+    {
+        if (null === $piece) {
+            return null;
+        }
+        try {
+            $piece->getId();
+
+            return $piece;
+        } catch (EntityNotFoundException) {
+            return null;
+        }
+    }
+
     private function normalizeCustomFieldValue(CustomFieldValue $value): array
     {
         $customField = $value->getCustomField();