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/utils/parent_registration_payload.dart

@@ -166,7 +166,6 @@ class ParentRegistrationPayload {
   static Map<String, dynamic> _childToJson(ChildData c, int index, String parentNom) {
     final map = <String, dynamic>{
       'genre': apiGenres.contains(c.genre) ? c.genre : 'Autre',
-      'grossesse_multiple': c.multipleBirth,
       'consent_photo': c.photoConsent,
     };
 

frontend/lib/utils/reprise_mapper.dart

@@ -44,7 +44,6 @@ class RepriseMapper {
       dob: dob,
       genre: e.gender ?? '',
       photoConsent: e.consentPhoto,
-      multipleBirth: e.estMultiple,
       isUnbornChild: isUnborn,
       cardColor: _childCardColors[index % _childCardColors.length],
       repriseChildId: e.id,