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/Configurations/SiteConfiguration.cs

@@ -27,6 +27,7 @@ public class SiteConfiguration : IEntityTypeConfiguration<Site>
         builder.Property(s => s.NodeBAddress).HasMaxLength(500);
         builder.Property(s => s.GrpcNodeAAddress).HasMaxLength(500);
         builder.Property(s => s.GrpcNodeBAddress).HasMaxLength(500);
+        builder.Property(s => s.ArtifactRevisionHash).HasMaxLength(100);
 
         builder.HasIndex(s => s.Name).IsUnique();
         builder.HasIndex(s => s.SiteIdentifier).IsUnique();

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Migrations/20260914173147_AddSiteArtifactRevision.cs

@@ -0,0 +1,40 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddSiteArtifactRevision : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "ArtifactRevisionHash",
+                table: "Sites",
+                type: "nvarchar(100)",
+                maxLength: 100,
+                nullable: true);
+
+            migrationBuilder.AddColumn<DateTimeOffset>(
+                name: "ArtifactsDeployedAt",
+                table: "Sites",
+                type: "datetimeoffset",
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "ArtifactRevisionHash",
+                table: "Sites");
+
+            migrationBuilder.DropColumn(
+                name: "ArtifactsDeployedAt",
+                table: "Sites");
+        }
+    }
+}

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/Migrations/ScadaBridgeDbContextModelSnapshot.cs

@@ -1379,6 +1379,13 @@ namespace ZB.MOM.WW.ScadaBridge.ConfigurationDatabase.Migrations
 
                     SqlServerPropertyBuilderExtensions.UseIdentityColumn(b.Property<int>("Id"));
 
+                    b.Property<string>("ArtifactRevisionHash")
+                        .HasMaxLength(100)
+                        .HasColumnType("nvarchar(100)");
+
+                    b.Property<DateTimeOffset?>("ArtifactsDeployedAt")
+                        .HasColumnType("datetimeoffset");
+
                     b.Property<string>("Description")
                         .HasMaxLength(2000)
                         .HasColumnType("nvarchar(2000)");

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)
     {