Fix self-referencing constant NO_SNAPSHOT_MSG in AdminController

The constant was incorrectly defined as self::NO_SNAPSHOT_MSG = self::NO_SNAPSHOT_MSG
causing a PHP fatal error. Replace with the actual string value.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Controller/AdminController.php

@@ -37,7 +37,7 @@ class AdminController extends AbstractController
 {
     private const DQL_STATUS_PAID = 'o.status = :paid';
     private const DQL_EXCLUDE_INVITATIONS = 'o.isInvitation = false OR o.isInvitation IS NULL';
-    private const NO_SNAPSHOT_MSG = self::NO_SNAPSHOT_MSG;
+    private const NO_SNAPSHOT_MSG = 'Aucun snapshot disponible.';
 
     #[Route('', name: 'app_admin_dashboard')]
     public function dashboard(EntityManagerInterface $em, #[Autowire(service: 'stats.cache')] CacheInterface $cache): Response