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/virtual_drone/main.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import os
 import random
 import select
 import time
@@ -108,6 +109,10 @@ def run(cfg: Config) -> None:
         writer.seal()
     print(f"[{cfg.drone_id}] done: {frames_sent} state frames sent, "
           f"{len(peers_seen)} peers seen {sorted(peers_seen)}; sealed to {root}")
+    if os.environ.get("KEEP_ALIVE", "0") == "1":
+        print(f"[{cfg.drone_id}] KEEP_ALIVE=1 — idle after seal (no pod restart churn)")
+        while True:
+            time.sleep(3600)
 
 
 if __name__ == "__main__":