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/models/enfant_admin_model.dart

@@ -12,7 +12,6 @@ class EnfantAdminModel {
   final String status;
   final String? photoUrl;
   final bool consentPhoto;
-  final bool isMultiple;
   final List<EnfantParentLink> parentLinks;
   /// Flag API #157 (sinon déduit de [parentLinks]).
   final bool? sansResponsable;
@@ -27,7 +26,6 @@ class EnfantAdminModel {
     required this.status,
     this.photoUrl,
     this.consentPhoto = false,
-    this.isMultiple = false,
     this.parentLinks = const [],
     this.sansResponsable,
   });
@@ -56,7 +54,6 @@ class EnfantAdminModel {
     String? status,
     String? photoUrl,
     bool? consentPhoto,
-    bool? isMultiple,
     List<EnfantParentLink>? parentLinks,
     bool? sansResponsable,
   }) {
@@ -70,7 +67,6 @@ class EnfantAdminModel {
       status: status ?? this.status,
       photoUrl: photoUrl ?? this.photoUrl,
       consentPhoto: consentPhoto ?? this.consentPhoto,
-      isMultiple: isMultiple ?? this.isMultiple,
       parentLinks: parentLinks ?? this.parentLinks,
       sansResponsable: sansResponsable ?? this.sansResponsable,
     );
@@ -111,8 +107,6 @@ class EnfantAdminModel {
       ),
       photoUrl: photoUrl,
       consentPhoto: consentPhoto,
-      isMultiple: _parseBool(json['is_multiple']) ||
-          _parseBool(json['est_multiple']),
       parentLinks: links,
       sansResponsable: sansResponsable,
     );
@@ -127,7 +121,6 @@ class EnfantAdminModel {
       if (birthDate != null && birthDate!.isNotEmpty) 'birth_date': birthDate,
       if (dueDate != null && dueDate!.isNotEmpty) 'due_date': dueDate,
       'consent_photo': consentPhoto,
-      'is_multiple': isMultiple,
     };
   }