feat(normalization) : drop structure and productIds JSON columns

- Remove Composant.structure property, getter/setter
- Remove Piece.productIds property, setProductIds()
- Update fixtures to remove dropped columns
- Add migrations to drop both columns

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Service/ModelTypeCategoryConversionService.php

@@ -341,8 +341,8 @@ final class ModelTypeCategoryConversionService
     {
         // 1. Insert into pieces from composants
         $count = $this->connection->executeStatement(
-            'INSERT INTO pieces (id, name, reference, prix, productids, typepieceid, productid, createdat, updatedat)
-             SELECT id, name, reference, prix, NULL, typecomposantid, productid, createdat, updatedat
+            'INSERT INTO pieces (id, name, reference, prix, typepieceid, productid, createdat, updatedat)
+             SELECT id, name, reference, prix, typecomposantid, productid, createdat, updatedat
              FROM composants
              WHERE typecomposantid = :id',
             ['id' => $modelTypeId],