feat(sites): record the artifact revision last deployed to a site

Site.ArtifactRevisionHash / ArtifactsDeployedAt (nullable, migration
AddSiteArtifactRevision) so the Sites page can tell whether a site's
system-wide artifacts are current. Not transported. Adds the
GetDeployedRevisionHashesAsync projection for the fleet-wide staleness batch.

Claude-Session: https://claude.ai/code/session_01Y5ef34iUUMjmwnMpL9xgmB

src/ZB.MOM.WW.ScadaBridge.Commons/Entities/Sites/Site.cs

@@ -18,6 +18,18 @@ public class Site
     public string? GrpcNodeAAddress { get; set; }
     /// <summary>gRPC endpoint for site node B used by the central SiteStreamGrpcClient.</summary>
     public string? GrpcNodeBAddress { get; set; }
+    /// <summary>
+    /// Revision hash (<c>ArtifactRevisionHashService</c>) of the per-site artifact set last
+    /// deployed to this site successfully; null = artifacts never deployed. Environment-specific
+    /// deploy state — deliberately NOT carried by Transport bundles (like <c>Instance.State</c>).
+    /// </summary>
+    public string? ArtifactRevisionHash { get; set; }
+    /// <summary>
+    /// UTC instant at which the artifact set identified by <see cref="ArtifactRevisionHash"/> was
+    /// last deployed to this site successfully; null = artifacts never deployed. Environment-specific
+    /// deploy state — deliberately NOT carried by Transport bundles (like <c>Instance.State</c>).
+    /// </summary>
+    public DateTimeOffset? ArtifactsDeployedAt { get; set; }
 
     /// <summary>
     /// Initializes a new site with the required name and identifier.