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/common/admin_child_detail_modal.dart

@@ -55,7 +55,6 @@ class _AdminChildDetailModalState extends State<AdminChildDetailModal> {
   late String _status;
   late String? _gender;
   late bool _consentPhoto;
-  late bool _isMultiple;
   bool _dirty = false;
   bool _saving = false;
   bool _deleting = false;
@@ -142,7 +141,6 @@ class _AdminChildDetailModalState extends State<AdminChildDetailModal> {
       _gender = _normalizeGender(e?.gender, allowUnknown: _isUnborn);
     }
     _consentPhoto = e?.consentPhoto ?? false;
-    _isMultiple = e?.isMultiple ?? false;
     for (final c in [_prenomCtrl, _nomCtrl]) {
       c.addListener(_onNameChanged);
     }
@@ -383,7 +381,6 @@ class _AdminChildDetailModalState extends State<AdminChildDetailModal> {
           else if (_dateToIso(_birthCtrl.text) != null)
             'birth_date': _dateToIso(_birthCtrl.text),
           'consent_photo': _consentPhoto,
-          'is_multiple': _isMultiple,
         },
       );
       if (!mounted) return;
@@ -460,7 +457,6 @@ class _AdminChildDetailModalState extends State<AdminChildDetailModal> {
               'birth_date': _dateToIso(_birthCtrl.text),
           },
           'consent_photo': _consentPhoto,
-          'is_multiple': _isMultiple,
         },
       );
       if (!mounted) return;

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;