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/auth/dto/enfant-inscription.dto.ts

@@ -55,11 +55,6 @@ export class EnfantInscriptionDto {
   @IsString()
   photo_filename?: string;
 
-  @ApiProperty({ example: false, required: false, description: 'Grossesse multiple (jumeaux, triplés, etc.)' })
-  @IsOptional()
-  @IsBoolean()
-  grossesse_multiple?: boolean;
-
   @ApiProperty({
     example: true,
     required: false,