fix(metrics): correct cpu/disk% chart scaling, evict stale collector state, lighten nic query, drop polling & dead RRD code

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

frontend/src/features/instances/tabs/GraphsTab.tsx

@@ -235,7 +235,7 @@ export default function GraphsTab({ serverId, vmid, type, node }: Props) {
             data={points}
             dataKey="cpu"
             color="hsl(217, 91%, 60%)"
-            formatValue={(v) => `${(v * 100).toFixed(1)}%`}
+            formatValue={(v) => `${v.toFixed(1)}%`}
             unit="%"
           />
           <ChartCard
@@ -280,7 +280,7 @@ export default function GraphsTab({ serverId, vmid, type, node }: Props) {
             data={points}
             dataKey="diskpct"
             color="hsl(45, 93%, 47%)"
-            formatValue={(v) => `${(v * 100).toFixed(1)}%`}
+            formatValue={(v) => `${v.toFixed(1)}%`}
             unit="%"
           />
           <ChartCard