fix(dashboard): downsample history graphs + readable tooltip time

The agent reports every few seconds, so multi-hour history series were hundreds–
thousands of points — a dense, noisy line (esp. CPU). Bucket-average server-side
to ~120 points (keeps the shape, drops the noise) for both the history-graph
widget and the host-detail charts. Also fix the chart tooltip title showing the
raw epoch-ms (e.g. 1,781,720,471,459) — format it as HH:MM.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/host_agent/templates/widget_history.html

@@ -48,7 +48,10 @@
         // still show the real %, so the values stay honest.
         y: { beginAtZero: true, grace: "8%", ticks: { color: "#8a8a92", font: { size: 10 } }, grid: { color: "#30303a" } },
       },
-      plugins: { legend: { labels: { color: "#b8b8b0", boxWidth: 10, font: { size: 11 } } } },
+      plugins: {
+        legend: { labels: { color: "#b8b8b0", boxWidth: 10, font: { size: 11 } } },
+        tooltip: { callbacks: { title: function (items) { return items.length ? fmtTime(items[0].parsed.x) : ""; } } },
+      },
     },
   });
 })();