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

backend/src/MoShiro.Backend.Storage.Sqlite/SqliteMetricStore.cs

@@ -90,6 +90,11 @@ public sealed class SqliteMetricStore : IMetricStore
             c => c.Query($"SELECT MIN(window_start) FROM {level.TableName()} WHERE agent_id = @a AND series = @s", r => r.NullableLong(0), ("@a", agentId.Value), ("@s", series)).FirstOrDefault(),
             ct);
 
+    public ValueTask<long?> LatestAsync(MetricLevel level, NodeId agentId, string series, CancellationToken ct) =>
+        _db.ReadAsync(
+            c => c.Query($"SELECT MAX(window_start) FROM {level.TableName()} WHERE agent_id = @a AND series = @s", r => r.NullableLong(0), ("@a", agentId.Value), ("@s", series)).FirstOrDefault(),
+            ct);
+
     public ValueTask<IReadOnlyList<MetricRow>> ReadAgentsAsync(MetricLevel level, IReadOnlyCollection<NodeId> agentIds, string series, long fromMs, long toMs, CancellationToken ct) =>
         _db.ReadAsync<IReadOnlyList<MetricRow>>(
             c =>