fix(health-monitoring): resolve HealthMonitoring-015 — nullable LastReportReceivedAt

A heartbeat-registered site that has never sent a full report now has
LastReportReceivedAt = null instead of the year-0001 sentinel. TimestampDisplay
accepts DateTimeOffset? and renders null as a placeholder ('awaiting first
report') rather than a ~2000-year-stale date. Cross-module: HealthMonitoring +
CentralUI.

src/ScadaLink.HealthMonitoring/SiteHealthState.cs

@@ -21,10 +21,13 @@ public sealed record SiteHealthState
     public SiteHealthReport? LatestReport { get; init; }
 
     /// <summary>
-    /// Time the latest full <see cref="SiteHealthReport"/> was processed.
-    /// Used by the UI to surface report staleness during failover.
+    /// Time the latest full <see cref="SiteHealthReport"/> was processed, or
+    /// <c>null</c> if the site is known only via heartbeats and has not yet sent
+    /// a report. Used by the UI to surface report staleness during failover;
+    /// the <c>null</c> case must be rendered as "no report yet" rather than as a
+    /// timestamp (a <c>default</c> sentinel would display as year-0001).
     /// </summary>
-    public DateTimeOffset LastReportReceivedAt { get; init; }
+    public DateTimeOffset? LastReportReceivedAt { get; init; }
 
     /// <summary>
     /// Time the most recent signal of any kind (full report OR heartbeat) was