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.CentralUI.Tests/Shared/TimestampDisplayTests.cs

@@ -0,0 +1,50 @@
+using Bunit;
+using ScadaLink.CentralUI.Components.Shared;
+
+namespace ScadaLink.CentralUI.Tests.Shared;
+
+/// <summary>
+/// Regression tests for HealthMonitoring-015. A heartbeat-only registered site has
+/// a <c>null</c> <c>LastReportReceivedAt</c> ("no full report yet"). The health
+/// dashboard passes that value straight into <see cref="TimestampDisplay"/>, so the
+/// component's <c>Value</c> must accept <c>DateTimeOffset?</c> and render a
+/// <c>null</c> as a human-readable placeholder ("never") instead of the
+/// <c>DateTimeOffset.MinValue</c> year-0001 sentinel. Non-null callers must keep
+/// rendering the formatted timestamp exactly as before.
+/// </summary>
+public class TimestampDisplayTests : BunitContext
+{
+    [Fact]
+    public void Render_NonNullValue_ShowsFormattedTimestamp()
+    {
+        var value = new DateTimeOffset(2026, 5, 17, 14, 30, 45, TimeSpan.Zero);
+
+        var cut = Render<TimestampDisplay>(parameters => parameters
+            .Add(p => p.Value, (DateTimeOffset?)value)
+            .Add(p => p.Format, "HH:mm:ss"));
+
+        var span = cut.Find("span");
+        Assert.Equal(value.LocalDateTime.ToString("HH:mm:ss"), span.TextContent.Trim());
+        Assert.Contains("2026-05-17 14:30:45 UTC", span.GetAttribute("title")!);
+    }
+
+    [Fact]
+    public void Render_NullValue_ShowsNeverPlaceholder()
+    {
+        var cut = Render<TimestampDisplay>(parameters => parameters
+            .Add(p => p.Value, (DateTimeOffset?)null)
+            .Add(p => p.Format, "HH:mm:ss"));
+
+        Assert.Contains("never", cut.Markup, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void Render_NullValue_DoesNotRenderYear0001Sentinel()
+    {
+        var cut = Render<TimestampDisplay>(parameters => parameters
+            .Add(p => p.Value, (DateTimeOffset?)null));
+
+        // The year-0001 DateTimeOffset.MinValue sentinel must never reach the UI.
+        Assert.DoesNotContain("0001", cut.Markup);
+    }
+}

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()
     {