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-durand-rousseau-test.mjs

@@ -74,8 +74,7 @@ try {
         date_naissance: '2022-04-20',
         genre: 'F',
         photo_base64: toDataUri(chloeJpg),
-        photo_filename: 'chloe_rousseau.jpg',
-        grossesse_multiple: false,
+        photo_filename: 'chloe_rousseau.jpg'
       },
       {
         prenom: 'Hugo',
@@ -83,8 +82,7 @@ try {
         date_naissance: '2024-03-10',
         genre: 'H',
         photo_base64: toDataUri(hugoJpg),
-        photo_filename: 'hugo_rousseau.jpg',
-        grossesse_multiple: false,
+        photo_filename: 'hugo_rousseau.jpg'
       },
     ],
     presentation_dossier: presentationDossier,