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>

docker/php/Caddyfile

@@ -29,6 +29,12 @@
         php_server
     }
 
+    # Vector tiles (.pbf) — pas de fallback SPA, retourner 404 si absent
+    handle /tiles/* {
+        header Content-Type "application/x-protobuf"
+        file_server
+    }
+
     # SPA React : try static files first, fallback to index.html
     handle {
         try_files {path} /index.html