refactor(health): move ICentralHealthAggregator and SiteHealthState to Commons (ScadaBridge#71)

Interface goes to Commons.Interfaces.Services and the state record to
Commons.Messages.Health, so CentralCommunicationActor no longer needs the
HealthMonitoring namespace. Implementation stays in HealthMonitoring.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

src/ZB.MOM.WW.ScadaBridge.Commons/Interfaces/Repositories/IAuditLogRepository.cs

@@ -1,3 +1,4 @@
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Services;
 using ZB.MOM.WW.Audit;
 using ZB.MOM.WW.ScadaBridge.Commons.Types;
 using ZB.MOM.WW.ScadaBridge.Commons.Types.Audit;

src/ZB.MOM.WW.ScadaBridge.Commons/Interfaces/Services/ICentralHealthAggregator.cs

@@ -0,0 +1,54 @@
+using ZB.MOM.WW.ScadaBridge.Commons.Messages.Health;
+
+namespace ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Services;
+
+/// <summary>
+/// Interface for central-side health aggregation.
+/// Consumed by Central UI to display site health dashboards.
+/// </summary>
+public interface ICentralHealthAggregator
+{
+    /// <summary>
+    /// Processes an incoming health report from a site and updates the aggregated state.
+    /// </summary>
+    /// <param name="report">The health report received from the site.</param>
+    void ProcessReport(SiteHealthReport report);
+
+    /// <summary>
+    /// Bumps the last-seen timestamp for a site, keeping it marked online
+    /// between full 30s reports when heartbeats are arriving — protects against
+    /// the offline threshold firing on a transiently delayed report. Heartbeat
+    /// cadence is owned by the Cluster Infrastructure / <c>SiteCommunicationActor</c>
+    /// (the application-level heartbeat to central, sent every
+    /// <c>CommunicationOptions.TransportHeartbeatInterval</c> — 5s by default);
+    /// the 60s <c>HealthMonitoringOptions.OfflineTimeout</c> therefore
+    /// tolerates several missed heartbeats. A heartbeat for a site with no
+    /// aggregator state yet (e.g. just after a central restart/failover)
+    /// registers that site as online with no
+    /// <see cref="SiteHealthState.LatestReport"/>, so reachable sites are not
+    /// shown as "unknown" during the failover window.
+    /// </summary>
+    /// <param name="siteId">The string identifier of the site that sent the heartbeat.</param>
+    /// <param name="receivedAt">The UTC timestamp when the heartbeat was received.</param>
+    void MarkHeartbeat(string siteId, DateTimeOffset receivedAt);
+
+    /// <summary>Returns a snapshot of all currently tracked site health states.</summary>
+    /// <returns>A dictionary mapping site id to its current <see cref="SiteHealthState"/>.</returns>
+    IReadOnlyDictionary<string, SiteHealthState> GetAllSiteStates();
+    /// <summary>Returns the current health state for the specified site, or null if not tracked.</summary>
+    /// <param name="siteId">The string identifier of the site to look up.</param>
+    /// <returns>The <see cref="SiteHealthState"/> for the site, or null if not yet seen.</returns>
+    SiteHealthState? GetSiteState(string siteId);
+
+    /// <summary>
+    /// Removes tracked sites not in <paramref name="knownSiteIds"/> (i.e. deleted
+    /// from configuration). The synthetic central id is always retained.
+    /// Self-healing: called from the CentralCommunicationActor's periodic site
+    /// refresh, so a deleted site disappears within one refresh interval without
+    /// needing a dedicated deletion event. Without this the in-memory state only
+    /// ever grew, leaving a deleted site as a permanently-offline dashboard tile
+    /// (and a live KPI sample source) forever.
+    /// </summary>
+    /// <param name="knownSiteIds">The identifiers of all currently-configured sites.</param>
+    void PruneUnknownSites(IReadOnlyCollection<string> knownSiteIds);
+}