fix(query): anchor=latest follows the newest sample of any level, not just the 1m rows [CI]

backend/src/MoShiro.Backend.Application/Query/RowLoader.cs

@@ -45,8 +45,19 @@ public sealed class RowLoader
             return at + 1000;
         }
 
-        IReadOnlyList<MetricRow> rows = await _metrics.ReadAsync(MetricLevel.M1, agent, series, nowMs - (7 * Steps.Day), nowMs + 60_000, ct);
-        return rows.Count == 0 ? null : rows.Max(r => r.WindowStart) + 60_000;
+        // 热层空了（Backend 刚重启、热层还没攒起来）就看持久层：各层里最晚的窗口取最大。只看 1 分钟层会落后——它比 5 分钟层更新得慢时，
+        // 窗口就被定在好几个小时以前，页面显示的全是旧数据。
+        long? latest = null;
+        foreach (MetricLevel level in new[] { MetricLevel.S10, MetricLevel.M1, MetricLevel.M5 })
+        {
+            if (await _metrics.LatestAsync(level, agent, series, ct) is { } start)
+            {
+                long end = Math.Min(start + level.WidthMs(), nowMs);
+                latest = latest is null ? end : Math.Max(latest.Value, end);
+            }
+        }
+
+        return latest;
     }
 
     /// <summary>