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/CentralHealthAggregator.cs

@@ -118,12 +118,14 @@ public class CentralHealthAggregator : BackgroundService, ICentralHealthAggregat
             if (!_siteStates.TryGetValue(siteId, out var existing))
             {
                 // Unknown site — register it as online, awaiting its first
-                // full report. LatestReport stays null until ProcessReport runs.
+                // full report. LatestReport and LastReportReceivedAt both stay
+                // null until ProcessReport runs — "no report yet" is an explicit
+                // nullable state, not a year-0001 sentinel the UI must special-case.
                 var registered = new SiteHealthState
                 {
                     SiteId = siteId,
                     LatestReport = null,
-                    LastReportReceivedAt = default,
+                    LastReportReceivedAt = null,
                     LastHeartbeatAt = receivedAt,
                     LastSequenceNumber = 0,
                     IsOnline = true