Do the doctrine definitions of the master preview attachment in the sub classes

This makes lazy fetching works and saves some db queries.

src/Entity/Base/MasterAttachmentTrait.php

@@ -33,10 +33,10 @@ trait MasterAttachmentTrait
 {
     /**
      * @var Attachment|null
+     * Mapping is done in the subclasses (e.g. Part), like with the attachments.
+     * If this is done here (which is possible in theory), the attachment is not lazy loaded anymore, which causes unnecessary overhead.
      */
     #[Assert\Expression('value == null or value.isPicture()', message: 'part.master_attachment.must_be_picture')]
-    #[ORM\ManyToOne(targetEntity: Attachment::class)]
-    #[ORM\JoinColumn(name: 'id_preview_attachment', onDelete: 'SET NULL')]
     protected ?Attachment $master_picture_attachment = null;
 
     /**