feat: ingestion automatique toutes les 6h + indicateur fraîcheur UI

- Symfony Scheduler (MainSchedule) : cron 0,6,12,18h UTC
- IngestionScheduleMessage + IngestionScheduleHandler : pipeline complet
  ingestion → forecasts → impact scores pour toutes les zones Antilles
- messenger.yaml : transport scheduler_main + async Redis (rs971_async)
- entrypoint.sh : 2 workers en background (scheduler_main + async)
- GET /api/status : date/heure de la dernière observation
- SargassesMap : badge "Données du JJ/MM à HH:MM" en bas à gauche

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/Controller/StatusController.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace App\Controller;
+
+use Doctrine\DBAL\Connection;
+use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
+use Symfony\Component\HttpFoundation\JsonResponse;
+use Symfony\Component\Routing\Attribute\Route;
+
+/**
+ * GET /api/status — fraîcheur des données pour l'indicateur UI
+ */
+#[Route('/api/status', name: 'api_status', methods: ['GET'])]
+class StatusController extends AbstractController
+{
+    public function __construct(private Connection $connection) {}
+
+    public function __invoke(): JsonResponse
+    {
+        $row = $this->connection->fetchAssociative(
+            'SELECT detected_at, source, coverage_area
+             FROM sargassum_observation
+             ORDER BY detected_at DESC LIMIT 1'
+        );
+
+        return $this->json([
+            'lastObservation' => $row !== false ? [
+                'detectedAt'   => $row['detected_at'],
+                'source'       => $row['source'],
+                'coverageKm2'  => $row['coverage_area'] !== null ? (float) $row['coverage_area'] : null,
+            ] : null,
+        ]);
+    }
+}