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>

docs/99_REGLES-CODAGE.md

@@ -276,9 +276,6 @@ export class Enfants {
   @Column({ name: 'consentement_photo', type: 'boolean', default: false })
   consentementPhoto: boolean;
 
-  @Column({ name: 'est_multiple', type: 'boolean', default: false })
-  estMultiple: boolean;
-
   @Column({
     type: 'enum',
     enum: StatutEnfantType,