feat(#152/#155): cleanup est_multiple + dashboard sans préfixe Admin (squash develop).

Suppression complète grossesse multiple / est_multiple (BDD, API, front).
Rename option C : widgets partagés et panels staff sous widgets/dashboard/,
AdminManagementWidget seul restant dans widgets/admin/.

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,