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/dto/enfants_response.dto.ts

@@ -29,9 +29,6 @@ export class EnfantResponseDto {
   @ApiProperty({ example: false })
   consent_photo: boolean;
 
-  @ApiProperty({ example: false })
-  is_multiple: boolean;
-
   @ApiProperty({ example: 'UUID-parent' })
   parent_id: string;
 }