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>

frontend/lib/widgets/admin/parent_dossier_wizard.dart

@@ -1466,7 +1466,6 @@ class _ParentDossierWizardState extends State<ParentDossierWizard> {
     final map = <String, dynamic>{
       'genre': c.genre,
       'consent_photo': true,
-      'grossesse_multiple': false,
     };
 
     if (prenom.length >= 2) {
@@ -1723,7 +1722,6 @@ class _ParentDossierWizardState extends State<ParentDossierWizard> {
       'status': status,
       'gender': gender,
       'consent_photo': true,
-      'is_multiple': false,
     };
     if (prenom.length >= 2) map['first_name'] = prenom;
     if (nom.length >= 2) map['last_name'] = nom;