feat(comments) : add file attachments on comments

Comments can now have documents attached via multipart/form-data upload.
New endpoint GET /api/documents/comment/{id} to list a comment's files.
Document entity gains a comment relation with cascade remove.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/State/DocumentUploadProcessor.php

@@ -109,6 +109,7 @@ final class DocumentUploadProcessor implements ProcessorInterface
             'pieceId'     => 'Piece',
             'productId'   => 'Product',
             'siteId'      => 'Site',
+            'commentId'   => 'Comment',
         ];
 
         foreach ($relationMap as $field => $entityName) {