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>

backend/src/routes/enfants/enfants.service.ts

@@ -92,7 +92,6 @@ export class EnfantsService {
       photo_url: photoUrl,
       consent_photo: !!dto.consent_photo,
       consent_photo_at: consentAt,
-      is_multiple: !!dto.is_multiple,
     });
     await this.childrenRepository.save(child);