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/screens/auth/parent_register_step3_screen.dart

@@ -121,7 +121,6 @@ class _ParentRegisterStep3ScreenState extends State<ParentRegisterStep3Screen> {
         dob: '',
         isUnbornChild: false,
         photoConsent: false,
-        multipleBirth: false,
         cardColor: cardColor,
       );
       registrationData.addChild(newChild);