feat(host): add NodeName to NodeOptions + INodeIdentityProvider

- NodeName: semantic role-within-cluster identifier (node-a/node-b on sites,
  central-a/central-b on central). Bound from ScadaLink:Node:NodeName.
- INodeIdentityProvider exposes the trimmed name (null if unconfigured) so
  downstream audit writers can stamp the new SourceNode column.

src/ScadaLink.Commons/Interfaces/Services/INodeIdentityProvider.cs

@@ -0,0 +1,22 @@
+namespace ScadaLink.Commons.Interfaces.Services;
+
+/// <summary>
+/// Surfaces the local node's semantic role-within-cluster name so downstream
+/// audit writers can stamp it on the SourceNode column.
+/// </summary>
+/// <remarks>
+/// Conventional values follow the pattern <c>node-a</c>/<c>node-b</c> on site
+/// nodes and <c>central-a</c>/<c>central-b</c> on central nodes. The value is
+/// a free-form operator-supplied label — there is no enforced format. When the
+/// configuration value is missing, empty, or whitespace, implementations
+/// return <c>null</c> so audit writers can persist NULL rather than an empty
+/// string.
+/// </remarks>
+public interface INodeIdentityProvider
+{
+    /// <summary>
+    /// The configured semantic node name, trimmed of surrounding whitespace.
+    /// <c>null</c> when unconfigured.
+    /// </summary>
+    string? NodeName { get; }
+}

src/ScadaLink.Host/NodeIdentityProvider.cs

@@ -0,0 +1,20 @@
+using Microsoft.Extensions.Options;
+using ScadaLink.Commons.Interfaces.Services;
+
+namespace ScadaLink.Host;
+
+/// <summary>
+/// Binds <see cref="INodeIdentityProvider"/> to <see cref="NodeOptions.NodeName"/>.
+/// Empty or whitespace values are normalised to <c>null</c>; otherwise the value
+/// is returned trimmed.
+/// </summary>
+internal sealed class NodeIdentityProvider : INodeIdentityProvider
+{
+    public string? NodeName { get; }
+
+    public NodeIdentityProvider(IOptions<NodeOptions> nodeOptions)
+    {
+        var configured = nodeOptions.Value.NodeName;
+        NodeName = string.IsNullOrWhiteSpace(configured) ? null : configured.Trim();
+    }
+}

src/ScadaLink.Host/NodeOptions.cs

@@ -4,6 +4,14 @@ public class NodeOptions
 {
     public string Role { get; set; } = string.Empty;
     public string NodeHostname { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Operator-configured semantic node name used to stamp the SourceNode
+    /// column on audit rows. Conventional values are <c>node-a</c>/<c>node-b</c>
+    /// on site nodes and <c>central-a</c>/<c>central-b</c> on central nodes,
+    /// but the value is a free-form label — no validation is enforced.
+    /// </summary>
+    public string NodeName { get; set; } = string.Empty;
     public string? SiteId { get; set; }
     public int RemotingPort { get; set; } = 8081;
     public int GrpcPort { get; set; } = 8083;

src/ScadaLink.Host/ScadaLink.Host.csproj

@@ -7,6 +7,10 @@
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>
 
+  <ItemGroup>
+    <InternalsVisibleTo Include="ScadaLink.Host.Tests" />
+  </ItemGroup>
+
   <ItemGroup>
     <PackageReference Include="Akka.Cluster.Hosting" />
     <PackageReference Include="Akka.Cluster.Tools" />

src/ScadaLink.Host/SiteServiceRegistration.cs

@@ -1,6 +1,7 @@
 using ScadaLink.AuditLog;
 using ScadaLink.ClusterInfrastructure;
 using ScadaLink.Communication;
+using ScadaLink.Commons.Interfaces.Services;
 using ScadaLink.DataConnectionLayer;
 using ScadaLink.ExternalSystemGateway;
 using ScadaLink.HealthMonitoring;
@@ -96,5 +97,10 @@ public static class SiteServiceRegistration
         services.Configure<HealthMonitoringOptions>(config.GetSection("ScadaLink:HealthMonitoring"));
         services.Configure<NotificationOptions>(config.GetSection("ScadaLink:Notification"));
         services.Configure<LoggingOptions>(config.GetSection("ScadaLink:Logging"));
+
+        // Audit Log (#23) — exposes ScadaLink:Node:NodeName to downstream audit
+        // writers so they can stamp the SourceNode column. Registered here in
+        // shared bootstrap because every node (central + site) needs it.
+        services.AddSingleton<INodeIdentityProvider, NodeIdentityProvider>();
     }
 }

tests/ScadaLink.Host.Tests/NodeIdentityProviderTests.cs

@@ -0,0 +1,50 @@
+using Microsoft.Extensions.Options;
+using ScadaLink.Commons.Interfaces.Services;
+
+namespace ScadaLink.Host.Tests;
+
+/// <summary>
+/// Tests for NodeIdentityProvider — surfaces the operator-configured semantic
+/// node name (e.g. node-a / node-b / central-a / central-b) used by downstream
+/// audit writers to stamp the SourceNode column.
+/// </summary>
+public class NodeIdentityProviderTests
+{
+    private static INodeIdentityProvider BuildProvider(string nodeName)
+    {
+        var options = Options.Create(new NodeOptions { NodeName = nodeName });
+        return new NodeIdentityProvider(options);
+    }
+
+    [Fact]
+    public void NodeIdentityProvider_returns_configured_NodeName()
+    {
+        var provider = BuildProvider("central-a");
+
+        Assert.Equal("central-a", provider.NodeName);
+    }
+
+    [Fact]
+    public void NodeIdentityProvider_returns_null_when_NodeName_unset()
+    {
+        var provider = BuildProvider(string.Empty);
+
+        Assert.Null(provider.NodeName);
+    }
+
+    [Fact]
+    public void NodeIdentityProvider_returns_null_when_NodeName_whitespace()
+    {
+        var provider = BuildProvider("   ");
+
+        Assert.Null(provider.NodeName);
+    }
+
+    [Fact]
+    public void NodeIdentityProvider_trims_whitespace()
+    {
+        var provider = BuildProvider("  node-a  ");
+
+        Assert.Equal("node-a", provider.NodeName);
+    }
+}