perf: bound lake scans and add platform Grafana dashboard

Explorer and exporter were scanning the full Parquet lake every 1–5s
(~2000 files, 200MB+), driving ~2.2 CPU cores. Limit metrics to the
last N flights, cache DuckDB views and the partition tree, slow live
polls to 2s, and keep drones alive after seal to stop restart churn.

Add node-exporter, scan-duration metrics, and a Swarm Platform Grafana
dashboard for node CPU/memory and scan health.

simulator/docker-compose.yml

@@ -24,7 +24,8 @@ services:
     command: ["python", "monitoring/exporter.py"]
     environment:
       DATA_DIR: /data
-      SCAN_INTERVAL_S: "5"
+      SCAN_INTERVAL_S: "30"
+      METRIC_FLIGHT_WINDOW: "5"
     volumes:
       - ${SWARM_T1_DIR:-../var/t1}:/data:ro
       - ./monitoring:/app/monitoring:ro
@@ -37,6 +38,9 @@ services:
     command: ["python", "explorer/server.py"]
     environment:
       DATA_DIR: /data
+      VIEW_REFRESH_S: "30"
+      TREE_CACHE_S: "15"
+      METRIC_FLIGHT_WINDOW: "5"
     volumes:
       - ${SWARM_T1_DIR:-../var/t1}:/data:ro
       - ./explorer:/app/explorer:ro