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.

tests/ScadaLink.HealthMonitoring.Tests/CentralHealthAggregatorTests.cs

@@ -240,6 +240,43 @@ public class CentralHealthAggregatorTests
         Assert.Equal(now, state.LastHeartbeatAt);
     }
 
+    /// <summary>
+    /// Regression test for HealthMonitoring-015. A heartbeat-only registered site
+    /// has never processed a full report, so <see cref="SiteHealthState.LastReportReceivedAt"/>
+    /// must be <c>null</c> — not the <c>DateTimeOffset.MinValue</c> (year-0001)
+    /// sentinel that the UI would otherwise render as a ~2000-year-stale timestamp.
+    /// The "no report yet" signal must be an explicit nullable state, consistent
+    /// with <see cref="SiteHealthState.LatestReport"/>.
+    /// </summary>
+    [Fact]
+    public void MarkHeartbeat_RegistersUnknownSite_WithNullLastReportReceivedAt()
+    {
+        _aggregator.MarkHeartbeat("site-new", _timeProvider.GetUtcNow());
+
+        var state = _aggregator.GetSiteState("site-new");
+        Assert.NotNull(state);
+        Assert.Null(state.LastReportReceivedAt);
+    }
+
+    /// <summary>
+    /// Regression test for HealthMonitoring-015. Once a full report is processed
+    /// for a heartbeat-registered site, <see cref="SiteHealthState.LastReportReceivedAt"/>
+    /// becomes a real (non-null) instant.
+    /// </summary>
+    [Fact]
+    public void ProcessReport_SetsLastReportReceivedAt_ForHeartbeatRegisteredSite()
+    {
+        _aggregator.MarkHeartbeat("site-new", _timeProvider.GetUtcNow());
+        _timeProvider.Advance(TimeSpan.FromSeconds(5));
+        var reportTime = _timeProvider.GetUtcNow();
+
+        _aggregator.ProcessReport(MakeReport("site-new", 1));
+
+        var state = _aggregator.GetSiteState("site-new");
+        Assert.NotNull(state);
+        Assert.Equal(reportTime, state.LastReportReceivedAt);
+    }
+
     [Fact]
     public void MarkHeartbeat_KeepsSiteOnline_BetweenReports()
     {