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/EventSubscriber/PieceProductSyncSubscriber.php

@@ -12,7 +12,7 @@ use Doctrine\ORM\Event\PreUpdateEventArgs;
 use Doctrine\ORM\Events;
 
 /**
- * Keep the legacy single product relation in sync with the new productIds array.
+ * Keep the legacy single product relation in sync with the ManyToMany products collection.
  */
 final class PieceProductSyncSubscriber implements EventSubscriber
 {