feat: phase A — backup B2, healthcheck /api/status, vector tiles Tippecanoe

- A1: scripts/backup-postgres.sh — pg_dump quotidien compressé → Backblaze B2 (rclone), rétention 30j
- A2: StatusController retourne HTTP 503 + healthy/alertReason si dernière observation > 6h
- A4: Dockerfile installe tippecanoe, GenerateTilesCommand génère 5 tilesets (obs + h6/12/24/48), Caddyfile sert /tiles/* sans fallback SPA, SargassesMap.jsx passe en sources vector tiles statiques
- chore: backend/public/assets/ ajouté au .gitignore (build artifacts)
- chore: setup vitest frontend + ImpactScoreServiceTest (session précédente)

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

backend/src/Controller/StatusController.php

@@ -29,7 +29,27 @@ class StatusController extends AbstractController
              ORDER BY requested_at DESC LIMIT 1"
         );
 
+        // Healthcheck : données fraîches si dernière observation < 6h
+        $healthy = false;
+        $alertReason = null;
+
+        if ($obs !== false && $obs['detected_at'] !== null) {
+            $detectedAt = new \DateTimeImmutable($obs['detected_at'], new \DateTimeZone('UTC'));
+            $now        = new \DateTimeImmutable('now', new \DateTimeZone('UTC'));
+            $ageSeconds = $now->getTimestamp() - $detectedAt->getTimestamp();
+
+            if ($ageSeconds <= 6 * 3600) {
+                $healthy = true;
+            } else {
+                $alertReason = 'stale_data';
+            }
+        } else {
+            $alertReason = 'no_data';
+        }
+
         return $this->json([
+            'healthy'     => $healthy,
+            'alertReason' => $alertReason,
             'lastObservation' => $obs !== false ? [
                 'detectedAt'  => $obs['detected_at'],
                 'source'      => $obs['source'],
@@ -43,6 +63,6 @@ class StatusController extends AbstractController
                 'finishedAt'  => $job['processed_at'],
                 'retryCount'  => (int) $job['retry_count'],
             ] : null,
-        ]);
+        ], $healthy ? 200 : 503);
     }
 }