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.ConfigurationDatabase/Repositories/DeploymentManagerRepository.cs

@@ -294,6 +294,17 @@ public class DeploymentManagerRepository : IDeploymentManagerRepository
             .FirstOrDefaultAsync(s => s.InstanceId == instanceId, cancellationToken);
     }
 
+    /// <inheritdoc />
+    public async Task<IReadOnlyDictionary<int, string>> GetDeployedRevisionHashesAsync(CancellationToken cancellationToken = default)
+    {
+        // Projection only: ConfigurationJson is never materialised, so this stays cheap enough to
+        // read for the whole fleet in one round trip.
+        return await _dbContext.Set<DeployedConfigSnapshot>()
+            .AsNoTracking()
+            .Select(s => new { s.InstanceId, s.RevisionHash })
+            .ToDictionaryAsync(s => s.InstanceId, s => s.RevisionHash, cancellationToken);
+    }
+
     /// <inheritdoc />
     public async Task AddDeployedSnapshotAsync(DeployedConfigSnapshot snapshot, CancellationToken cancellationToken = default)
     {