fix: disable transaction on initial migration (CONCURRENTLY indexes)

backend/migrations/Version20260401000000.php

@@ -9,6 +9,11 @@ use Doctrine\Migrations\AbstractMigration;
 
 final class Version20260401000000 extends AbstractMigration
 {
+    public function isTransactional(): bool
+    {
+        return false;
+    }
+
     public function getDescription(): string
     {
         return 'Schéma initial — toutes les entités + extension PostGIS + index spatiaux';