feat(#152): suppression complète grossesse multiple / est_multiple.

Retire le champ partout : BDD (migration + seed), entity/DTO/services Nest,
modèles et payloads Flutter, scripts de test et docs. Back et front alignés
(forbidNonWhitelisted). Aucun fantôme de compat.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/scripts/register-parent-martin-test.mjs

@@ -46,8 +46,7 @@ const body = {
       date_naissance: '2023-02-15',
       genre: 'F',
       photo_base64: toDataUri(path.join(photosDir, 'martin-emma.png')),
-      photo_filename: 'emma_martin.png',
-      grossesse_multiple: true,
+      photo_filename: 'emma_martin.png'
     },
     {
       prenom: 'Noah',
@@ -55,8 +54,7 @@ const body = {
       date_naissance: '2023-02-15',
       genre: 'H',
       photo_base64: toDataUri(path.join(photosDir, 'martin-noah.png')),
-      photo_filename: 'noah_martin.png',
-      grossesse_multiple: true,
+      photo_filename: 'noah_martin.png'
     },
     {
       prenom: 'Léa',
@@ -64,8 +62,7 @@ const body = {
       date_naissance: '2023-02-15',
       genre: 'F',
       photo_base64: toDataUri(path.join(photosDir, 'martin-lea.png')),
-      photo_filename: 'lea_martin.png',
-      grossesse_multiple: true,
+      photo_filename: 'lea_martin.png'
     },
   ],
   presentation_dossier: presentationDossier,