fix(health): flag metrics-stale for never-reported sites via FirstSeenAt anchor — null LastReportReceivedAt no longer skips the check (plan R2-01 T8)

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/SiteHealthState.cs

@@ -40,6 +40,17 @@ public sealed record SiteHealthState
     /// </summary>
     public DateTimeOffset LastHeartbeatAt { get; init; }
 
+    /// <summary>
+    /// Gets the instant this site was first observed by the aggregator (via a
+    /// report or a heartbeat); <c>null</c> only for states not built by the
+    /// aggregator (hand-constructed fixtures). Anchors metrics-staleness for a
+    /// site that has NEVER delivered a report (review 01 round-2 N3): with
+    /// <see cref="LastReportReceivedAt"/> null the staleness check previously
+    /// skipped the site forever. <see cref="LastHeartbeatAt"/> cannot anchor
+    /// this — every heartbeat advances it.
+    /// </summary>
+    public DateTimeOffset? FirstSeenAt { get; init; }
+
     /// <summary>Gets the sequence number of the last accepted health report, used to reject out-of-order duplicates.</summary>
     public long LastSequenceNumber { get; init; }
     /// <summary>Gets a value indicating whether the site is currently considered online.</summary>