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/user_registration_data.dart

@@ -38,7 +38,6 @@ class ChildData {
   /// Valeurs API : `H`, `F`, `Autre` (GenreType backend). Vide tant que non choisi.
   String genre;
   bool photoConsent;
-  bool multipleBirth;
   bool isUnbornChild;
   File? imageFile;
   /// Octets de la photo (fiable à l’envoi API ; [imageFile] peut être absent sur le web).
@@ -55,7 +54,6 @@ class ChildData {
     this.dob = '',
     this.genre = '',
     this.photoConsent = false,
-    this.multipleBirth = false,
     this.isUnbornChild = false,
     this.imageFile,
     this.imageBytes,
@@ -70,7 +68,6 @@ class ChildData {
     String? dob,
     String? genre,
     bool? photoConsent,
-    bool? multipleBirth,
     bool? isUnbornChild,
     Object? imageFile = _unsetImage,
     Object? imageBytes = _unsetImageBytes,
@@ -84,7 +81,6 @@ class ChildData {
       dob: dob ?? this.dob,
       genre: genre ?? this.genre,
       photoConsent: photoConsent ?? this.photoConsent,
-      multipleBirth: multipleBirth ?? this.multipleBirth,
       isUnbornChild: isUnbornChild ?? this.isUnbornChild,
       imageFile: identical(imageFile, _unsetImage) ? this.imageFile : imageFile as File?,
       imageBytes: