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.

prototype/src/App.tsx

@@ -25,7 +25,7 @@ function displayAspect(): number {
   return Math.max(1, Math.min(MAX_ASPECT, q));
 }
 
-const LIVE_POLL_MS = 1000;
+const LIVE_POLL_MS = 2000;
 
 export default function App(): JSX.Element {
   const [droneCount, setDroneCount] = useState(8);