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/dossier_unifie.dart

@@ -185,7 +185,6 @@ class EnfantDossier {
   final String? dueDate;
   final String? photoUrl;
   final bool consentPhoto;
-  final bool estMultiple;
 
   EnfantDossier({
     required this.id,
@@ -197,7 +196,6 @@ class EnfantDossier {
     this.dueDate,
     this.photoUrl,
     this.consentPhoto = false,
-    this.estMultiple = false,
   });
 
   String get fullName => '${firstName ?? ''} ${lastName ?? ''}'.trim();
@@ -231,8 +229,6 @@ class EnfantDossier {
       photoUrl: resolvedPhoto,
       consentPhoto:
           json['consent_photo'] == true || json['consentPhoto'] == true,
-      estMultiple:
-          json['est_multiple'] == true || json['estMultiple'] == true,
     );
   }
 }