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/entities/children.entity.ts

@@ -63,9 +63,6 @@ export class Children {
   @Column({ type: 'timestamptz', nullable: true, name: 'date_consentement_photo' })
   consent_photo_at?: Date;
 
-  @Column({ default: false, name: 'est_multiple', type: 'boolean' })
-  is_multiple: boolean;
-
   // Lien via table de jointure enfants_parents
   @OneToMany(() => ParentsChildren, pc => pc.child)
   parentLinks: ParentsChildren[];