feat(localdb): fail-closed auth on the sync endpoint + replication health signal

Tasks 8 and 9 of the LocalDb Phase 1 adoption plan.

Task 8 - LocalDbSyncAuthInterceptor. The replication library's LocalDbSyncService
verifies nothing; inbound auth is explicitly the host's job. Without this,
anything able to reach a site node's gRPC port could stream arbitrary rows into
the consolidated site database - including OperationTracking, which central
reconciles from.

Scoped strictly to /localdb_sync.v1.LocalDbSync/; SiteStream shares the same
AddGrpc pipeline and passes through untouched. Fail-closed: with no
LocalDb:Replication:ApiKey configured NO sync stream is accepted, authenticated
or not. That is deliberate - "no key" is the default every site node ships with,
so treating it as "no auth required" would expose the endpoint on precisely the
most common configuration. Comparison is FixedTimeEquals over UTF-8 bytes.

All four server handler shapes are gated, not just unary: the sync RPC is a
bidirectional stream, so gating only the unary path would leave the real endpoint
open while every unary test still passed. There is a test for that.

Deviation from the plan: it specified Grpc.Core.Testing for the fake
ServerCallContext. That type ships in the retired native Grpc.Core package and
does not exist on the grpc-dotnet stack this solution uses; a minimal
FakeServerCallContext in the test file was the better trade than adding a dead
dependency.

Task 9 - ISyncStatus onto the site health report as LocalDbReplicationConnected
and LocalDbOplogBacklog, via a delegate-seam hosted service following the
AddSiteEventLogHealthMetricsBridge precedent (HealthMonitoring takes no reference
on the replication library). Both are additive init properties, so the
Akka-remoted SiteHealthReport constructor signature is untouched.

Both fields are nullable and the distinction is load-bearing:
  - null = the reporter has not run / replication is not wired ("no data");
  - false/0 = a real reading. On a node with no peer that IS the healthy
    default-OFF state, not an outage.
OplogBacklog is passed through nullable end-to-end because ISyncStatus returns
null when the poll fails - flattening it to 0 would report a replication pair
that cannot read its own oplog as perfectly healthy. The collector stores both
values as one tuple and CollectReport reads it once, so a torn read cannot pair a
fresh Connected with a stale backlog.

Verified: build 0 warnings; Host 307/307 (8 interceptor + 3 health tests new),
HealthMonitoring 97/97, Commons 684/684.

Claude-Session: https://claude.ai/code/session_01BL2Vu1ESDQ9SCN4gVKkdts

src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Health/SiteHealthReport.cs

@@ -73,6 +73,37 @@ public record SiteHealthReport(
     /// indicates a stuck/blocked script holding a dedicated thread.
     /// </summary>
     public double? ScriptOldestBusyAgeSeconds { get; init; }
+
+    // LocalDb 2-node replication of the consolidated site database (Phase 1).
+    // Additive init properties for the same reason as the scheduler gauges above:
+    // the positional constructor stays untouched. Refreshed on the site by
+    // LocalDbReplicationStatusReporter.
+
+    /// <summary>
+    /// Whether a replication sync session is currently running with the peer site node,
+    /// or <see langword="null"/> when replication is not wired on this node.
+    /// </summary>
+    /// <remarks>
+    /// Nullable on purpose, and <see langword="false"/> is NOT the same as null.
+    /// Replication ships default-OFF, so a site node with no peer configured reports
+    /// <see langword="false"/> — that is a healthy, expected state, not an outage. Only
+    /// a node that was configured with a peer and is reporting <see langword="false"/>
+    /// is degraded, and distinguishing those two cases is the operator's job, not this
+    /// field's.
+    /// </remarks>
+    public bool? LocalDbReplicationConnected { get; init; }
+
+    /// <summary>
+    /// Unacked replication oplog entries waiting to reach the peer, or
+    /// <see langword="null"/> when unknown.
+    /// </summary>
+    /// <remarks>
+    /// <b>Null means unknown, never zero.</b> The underlying provider returns null when
+    /// no backlog source is wired or the poll failed, and that must survive to the
+    /// operator: a failed read rendered as "0 backlog" would report a broken replication
+    /// pair as perfectly healthy. Collapsing null to 0 anywhere on this path is a bug.
+    /// </remarks>
+    public long? LocalDbOplogBacklog { get; init; }
 }
 
 /// <summary>

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/ISiteHealthCollector.cs

@@ -141,6 +141,25 @@ public interface ISiteHealthCollector
         // SiteHealthCollector overrides this with the Interlocked.Exchange store.
     }
 
+    /// <summary>
+    /// Replace the latest LocalDb replication status (peer-session connectivity and
+    /// unacked oplog backlog) used by the next <see cref="CollectReport"/> call.
+    /// Refreshed periodically by the <c>LocalDbReplicationStatusReporter</c> hosted
+    /// service. Point-in-time: values are NOT reset on <see cref="CollectReport"/>.
+    /// </summary>
+    /// <param name="connected">
+    /// Whether a sync session is currently running. <see langword="false"/> is the
+    /// normal state on a node with no peer configured — replication ships default-OFF.
+    /// </param>
+    /// <param name="oplogBacklog">
+    /// Unacked oplog entries, or <see langword="null"/> when unknown. Pass null through
+    /// unchanged: a failed poll rendered as 0 would report a broken pair as healthy.
+    /// </param>
+    void SetLocalDbReplicationStatus(bool connected, long? oplogBacklog)
+    {
+        // Default no-op so test fakes do not need to be updated.
+    }
+
     /// <summary>
     /// Replace the latest script-execution-scheduler gauges (queue depth, busy
     /// thread count, and age in seconds of the oldest in-flight script) used by

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/LocalDbReplicationStatusReporter.cs

@@ -0,0 +1,130 @@
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+
+namespace ZB.MOM.WW.ScadaBridge.HealthMonitoring;
+
+/// <summary>
+/// Site-side hosted service that periodically reads LocalDb replication status and pushes
+/// it into <see cref="ISiteHealthCollector"/>, so the next
+/// <see cref="ISiteHealthCollector.CollectReport"/> emits fresh
+/// <c>LocalDbReplicationConnected</c> / <c>LocalDbOplogBacklog</c> fields.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <b>Why delegates and not <c>ISyncStatus</c> directly.</b> Same reasoning as
+/// <see cref="SiteEventLogFailureCountReporter"/>: HealthMonitoring does not take a
+/// reference on the replication library. The Host site wiring captures the two reads as
+/// lambdas at registration time; this service only moves numbers.
+/// </para>
+/// <para>
+/// <b>Null backlog is preserved, never coerced to zero.</b> <c>ISyncStatus.OplogBacklog</c>
+/// is nullable precisely so a failed poll reads as "unknown" — rendering it as 0 would
+/// report a replication pair that cannot read its own oplog as perfectly healthy.
+/// </para>
+/// <para>
+/// <b>Cadence.</b> 30 s, matching <see cref="SiteEventLogFailureCountReporter"/> and
+/// <c>SiteAuditBacklogReporter</c>. Any exception during a probe is logged and swallowed;
+/// the next tick retries.
+/// </para>
+/// </remarks>
+public sealed class LocalDbReplicationStatusReporter : IHostedService, IDisposable
+{
+    /// <summary>Default poll cadence, matching the other site health bridges.</summary>
+    internal static readonly TimeSpan DefaultRefreshInterval = TimeSpan.FromSeconds(30);
+
+    private readonly Func<bool> _connectedProvider;
+    private readonly Func<long?> _oplogBacklogProvider;
+    private readonly ISiteHealthCollector _collector;
+    private readonly ILogger<LocalDbReplicationStatusReporter> _logger;
+    private readonly TimeSpan _refreshInterval;
+    private CancellationTokenSource? _cts;
+    private Task? _loop;
+
+    /// <summary>Initializes a new instance of <see cref="LocalDbReplicationStatusReporter"/>.</summary>
+    /// <param name="connectedProvider">Reads whether a peer sync session is currently running.</param>
+    /// <param name="oplogBacklogProvider">Reads the unacked oplog backlog, or null when unknown.</param>
+    /// <param name="collector">The site health collector receiving the snapshot.</param>
+    /// <param name="logger">Logger instance.</param>
+    /// <param name="refreshInterval">Poll interval override; defaults to 30 s.</param>
+    public LocalDbReplicationStatusReporter(
+        Func<bool> connectedProvider,
+        Func<long?> oplogBacklogProvider,
+        ISiteHealthCollector collector,
+        ILogger<LocalDbReplicationStatusReporter> logger,
+        TimeSpan? refreshInterval = null)
+    {
+        _connectedProvider = connectedProvider ?? throw new ArgumentNullException(nameof(connectedProvider));
+        _oplogBacklogProvider = oplogBacklogProvider ?? throw new ArgumentNullException(nameof(oplogBacklogProvider));
+        _collector = collector ?? throw new ArgumentNullException(nameof(collector));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+        _refreshInterval = refreshInterval ?? DefaultRefreshInterval;
+    }
+
+    /// <summary>Starts the polling loop, probing once immediately before entering the timed cycle.</summary>
+    /// <param name="ct">Cancellation token signalling host shutdown.</param>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    public Task StartAsync(CancellationToken ct)
+    {
+        var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
+        _cts = cts;
+        _loop = RunAsync(cts.Token);
+        return Task.CompletedTask;
+    }
+
+    /// <summary>Stops the polling loop.</summary>
+    /// <param name="ct">Cancellation token for the stop operation.</param>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    public async Task StopAsync(CancellationToken ct)
+    {
+        if (_cts is null) return;
+
+        await _cts.CancelAsync().ConfigureAwait(false);
+        if (_loop is not null)
+        {
+            // Await the loop so the reporter is quiescent before the host disposes the
+            // collector out from under it.
+            try { await _loop.ConfigureAwait(false); }
+            catch (OperationCanceledException) { /* expected on shutdown */ }
+        }
+    }
+
+    private async Task RunAsync(CancellationToken ct)
+    {
+        while (!ct.IsCancellationRequested)
+        {
+            Probe();
+
+            try
+            {
+                await Task.Delay(_refreshInterval, ct).ConfigureAwait(false);
+            }
+            catch (OperationCanceledException)
+            {
+                return;
+            }
+        }
+    }
+
+    /// <summary>
+    /// Reads both providers and pushes one snapshot onto the collector; never throws.
+    /// Public so a test can drive a single deterministic probe instead of starting the
+    /// service and waiting out the 30 s cadence.
+    /// </summary>
+    public void Probe()
+    {
+        try
+        {
+            _collector.SetLocalDbReplicationStatus(
+                _connectedProvider(), _oplogBacklogProvider());
+        }
+        catch (Exception ex)
+        {
+            // Health reporting must never take the site node down. The previous snapshot
+            // stays on the collector and the next tick retries.
+            _logger.LogWarning(ex, "Failed to read LocalDb replication status for the site health report.");
+        }
+    }
+
+    /// <inheritdoc />
+    public void Dispose() => _cts?.Dispose();
+}

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/ServiceCollectionExtensions.cs

@@ -21,6 +21,12 @@ public static class ServiceCollectionExtensions
     /// </summary>
     private sealed class SiteEventLogHealthMetricsBridgeMarker { }
 
+    /// <summary>
+    /// Sentinel marker for <see cref="AddLocalDbReplicationHealthBridge"/>'s idempotency
+    /// guard — same rationale as <see cref="SiteEventLogHealthMetricsBridgeMarker"/>.
+    /// </summary>
+    private sealed class LocalDbReplicationHealthBridgeMarker { }
+
     /// <summary>
     /// Register site-side health monitoring services (metric collection + periodic reporting).
     /// Call this on site nodes only. For central, call AddCentralHealthAggregation() instead.
@@ -153,6 +159,46 @@ public static class ServiceCollectionExtensions
         return services;
     }
 
+    /// <summary>
+    /// Bridge LocalDb replication status (peer connectivity + unacked oplog backlog) onto
+    /// the site health report. Must be called AFTER <c>AddSiteHealthMonitoring</c>
+    /// (registers <see cref="ISiteHealthCollector"/>) and after the replication engine is
+    /// registered. Idempotent via a marker sentinel.
+    /// </summary>
+    /// <remarks>
+    /// Delegates rather than an <c>ISyncStatus</c> parameter keep HealthMonitoring free of
+    /// a reference on the replication library, matching
+    /// <see cref="AddSiteEventLogHealthMetricsBridge"/>. Pass the backlog through as
+    /// nullable — coercing null to 0 would report an unreadable oplog as a healthy one.
+    /// </remarks>
+    /// <param name="services">The service collection to register into.</param>
+    /// <param name="connectedProvider">Given the root provider, returns a reader for "a sync session is running".</param>
+    /// <param name="oplogBacklogProvider">Given the root provider, returns a reader for the unacked backlog (null = unknown).</param>
+    /// <returns>The same <see cref="IServiceCollection"/> for chaining.</returns>
+    public static IServiceCollection AddLocalDbReplicationHealthBridge(
+        this IServiceCollection services,
+        Func<IServiceProvider, Func<bool>> connectedProvider,
+        Func<IServiceProvider, Func<long?>> oplogBacklogProvider)
+    {
+        ArgumentNullException.ThrowIfNull(services);
+        ArgumentNullException.ThrowIfNull(connectedProvider);
+        ArgumentNullException.ThrowIfNull(oplogBacklogProvider);
+
+        if (services.Any(d => d.ServiceType == typeof(LocalDbReplicationHealthBridgeMarker)))
+        {
+            return services;
+        }
+
+        services.AddSingleton<LocalDbReplicationHealthBridgeMarker>();
+        services.AddHostedService(sp => new LocalDbReplicationStatusReporter(
+            connectedProvider(sp),
+            oplogBacklogProvider(sp),
+            sp.GetRequiredService<ISiteHealthCollector>(),
+            sp.GetRequiredService<ILogger<LocalDbReplicationStatusReporter>>()));
+
+        return services;
+    }
+
     /// <summary>
     /// Register the <see cref="HealthMonitoringOptionsValidator"/>
     /// so a misconfigured <c>ScadaBridge:HealthMonitoring</c> section (zero/negative

src/ZB.MOM.WW.ScadaBridge.HealthMonitoring/SiteHealthCollector.cs

@@ -18,6 +18,10 @@ public class SiteHealthCollector : ISiteHealthCollector
     private int _auditRedactionFailures;
     private volatile SiteAuditBacklogSnapshot? _siteAuditBacklog;
     private long _siteEventLogWriteFailures;
+    // One volatile tuple rather than two independent fields: a torn read that paired a
+    // fresh Connected with a stale backlog would be indistinguishable from a real state.
+    // Null = the reporter has not yet run (or replication is not wired).
+    private volatile Tuple<bool, long?>? _localDbReplicationStatus;
     private readonly ConcurrentDictionary<string, ConnectionHealth> _connectionStatuses = new();
     private readonly ConcurrentDictionary<string, TagResolutionStatus> _tagResolutionCounts = new();
     private readonly ConcurrentDictionary<string, string> _connectionEndpoints = new();
@@ -94,6 +98,12 @@ public class SiteHealthCollector : ISiteHealthCollector
         Interlocked.Exchange(ref _siteEventLogWriteFailures, count);
     }
 
+    /// <inheritdoc />
+    public void SetLocalDbReplicationStatus(bool connected, long? oplogBacklog)
+    {
+        _localDbReplicationStatus = Tuple.Create(connected, oplogBacklog);
+    }
+
     /// <inheritdoc />
     public void UpdateConnectionHealth(string connectionName, ConnectionHealth health)
     {
@@ -219,6 +229,9 @@ public class SiteHealthCollector : ISiteHealthCollector
         var deadLetters = Interlocked.Exchange(ref _deadLetterCount, 0);
         var siteAuditWriteFailures = Interlocked.Exchange(ref _siteAuditWriteFailures, 0);
         var auditRedactionFailures = Interlocked.Exchange(ref _auditRedactionFailures, 0);
+        // Single read of the volatile tuple — two reads could straddle a reporter tick and
+        // pair a fresh Connected with a stale backlog.
+        var localDbReplication = _localDbReplicationStatus;
 
         // Snapshot current connection and tag resolution state
         var connectionStatuses = new Dictionary<string, ConnectionHealth>(_connectionStatuses);
@@ -259,7 +272,12 @@ public class SiteHealthCollector : ISiteHealthCollector
         {
             ScriptQueueDepth = Interlocked.CompareExchange(ref _scriptQueueDepth, 0, 0),
             ScriptBusyThreads = Interlocked.CompareExchange(ref _scriptBusyThreads, 0, 0),
-            ScriptOldestBusyAgeSeconds = ReadScriptOldestBusyAgeSeconds()
+            ScriptOldestBusyAgeSeconds = ReadScriptOldestBusyAgeSeconds(),
+            // Both fields come from the ONE snapshot read above. Null (the reporter has
+            // not run) leaves both report fields null — "no data", not "disconnected
+            // with an empty backlog".
+            LocalDbReplicationConnected = localDbReplication?.Item1,
+            LocalDbOplogBacklog = localDbReplication?.Item2
         };
     }
 }

src/ZB.MOM.WW.ScadaBridge.Host/LocalDbSyncAuthInterceptor.cs

@@ -0,0 +1,158 @@
+using System.Security.Cryptography;
+using System.Text;
+using Grpc.Core;
+using Grpc.Core.Interceptors;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.LocalDb.Replication;
+
+namespace ZB.MOM.WW.ScadaBridge.Host;
+
+/// <summary>
+/// Gates the LocalDb passive sync endpoint. The replication library deliberately leaves
+/// inbound authentication to the host — its <c>LocalDbSyncService</c> verifies nothing —
+/// so without this interceptor anything that can reach the site node's gRPC port could
+/// stream arbitrary rows into the consolidated site database.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <b>Scoped by method path.</b> Only calls under
+/// <c>/localdb_sync.v1.LocalDbSync/</c> are gated; every other method — notably the
+/// existing <c>SiteStream</c> service sharing this listener — passes through untouched.
+/// </para>
+/// <para>
+/// <b>Fail-closed.</b> With no <c>LocalDb:Replication:ApiKey</c> configured, NO sync
+/// stream is accepted, authenticated or not. That is the deliberate choice: the
+/// alternative — treating "no key" as "no auth required" — would silently expose the
+/// endpoint on exactly the default configuration every site node ships with. An operator
+/// enabling replication must set the same key on both nodes, which is already required
+/// for the initiator to dial out (<c>SyncBackgroundService</c> sends
+/// <c>Authorization: Bearer &lt;key&gt;</c>).
+/// </para>
+/// <para>
+/// Comparison is <see cref="CryptographicOperations.FixedTimeEquals"/> over UTF-8 bytes,
+/// so a wrong key cannot be recovered byte-by-byte from response timing. Length differences
+/// are unavoidably observable and are not sensitive.
+/// </para>
+/// </remarks>
+public sealed class LocalDbSyncAuthInterceptor : Interceptor
+{
+    private const string ServicePrefix = "/localdb_sync.v1.LocalDbSync/";
+    private const string AuthorizationHeader = "authorization";
+    private const string BearerPrefix = "Bearer ";
+
+    private readonly IOptions<ReplicationOptions> _options;
+    private readonly ILogger<LocalDbSyncAuthInterceptor> _logger;
+
+    /// <summary>Creates the interceptor.</summary>
+    /// <param name="options">Replication options; <c>ApiKey</c> is the expected bearer token.</param>
+    /// <param name="logger">Logger for denial diagnostics.</param>
+    public LocalDbSyncAuthInterceptor(
+        IOptions<ReplicationOptions> options,
+        ILogger<LocalDbSyncAuthInterceptor> logger)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+        ArgumentNullException.ThrowIfNull(logger);
+
+        _options = options;
+        _logger = logger;
+    }
+
+    /// <inheritdoc />
+    public override Task<TResponse> UnaryServerHandler<TRequest, TResponse>(
+        TRequest request,
+        ServerCallContext context,
+        UnaryServerMethod<TRequest, TResponse> continuation)
+    {
+        Authorize(context);
+        return continuation(request, context);
+    }
+
+    /// <inheritdoc />
+    public override Task DuplexStreamingServerHandler<TRequest, TResponse>(
+        IAsyncStreamReader<TRequest> requestStream,
+        IServerStreamWriter<TResponse> responseStream,
+        ServerCallContext context,
+        DuplexStreamingServerMethod<TRequest, TResponse> continuation)
+    {
+        Authorize(context);
+        return continuation(requestStream, responseStream, context);
+    }
+
+    /// <inheritdoc />
+    public override Task<TResponse> ClientStreamingServerHandler<TRequest, TResponse>(
+        IAsyncStreamReader<TRequest> requestStream,
+        ServerCallContext context,
+        ClientStreamingServerMethod<TRequest, TResponse> continuation)
+    {
+        Authorize(context);
+        return continuation(requestStream, context);
+    }
+
+    /// <inheritdoc />
+    public override Task ServerStreamingServerHandler<TRequest, TResponse>(
+        TRequest request,
+        IServerStreamWriter<TResponse> responseStream,
+        ServerCallContext context,
+        ServerStreamingServerMethod<TRequest, TResponse> continuation)
+    {
+        Authorize(context);
+        return continuation(request, responseStream, context);
+    }
+
+    /// <summary>
+    /// Throws <see cref="RpcException"/> with <see cref="StatusCode.PermissionDenied"/> if
+    /// this is a sync call that does not carry the configured bearer token. Non-sync calls
+    /// return immediately.
+    /// </summary>
+    private void Authorize(ServerCallContext context)
+    {
+        if (!context.Method.StartsWith(ServicePrefix, StringComparison.Ordinal))
+            return;
+
+        var expected = _options.Value.ApiKey;
+        if (string.IsNullOrEmpty(expected))
+        {
+            _logger.LogWarning(
+                "Rejected a LocalDb sync call to {Method}: no LocalDb:Replication:ApiKey is configured, " +
+                "so the passive sync endpoint is closed. Configure the same key on both nodes of the pair.",
+                context.Method);
+            throw new RpcException(new Status(
+                StatusCode.PermissionDenied,
+                "LocalDb sync is not accepting connections: no API key is configured on this node."));
+        }
+
+        var presented = ExtractBearerToken(context.RequestHeaders);
+        if (presented is null || !FixedTimeEquals(presented, expected))
+        {
+            _logger.LogWarning(
+                "Rejected a LocalDb sync call to {Method}: {Reason}.",
+                context.Method,
+                presented is null ? "no bearer token presented" : "bearer token did not match");
+            throw new RpcException(new Status(
+                StatusCode.PermissionDenied,
+                "LocalDb sync authentication failed."));
+        }
+    }
+
+    private static string? ExtractBearerToken(Metadata headers)
+    {
+        // gRPC lowercases header keys on the wire; compare case-insensitively anyway so a
+        // hand-built Metadata in a test behaves the same as a real request.
+        foreach (var entry in headers)
+        {
+            if (!string.Equals(entry.Key, AuthorizationHeader, StringComparison.OrdinalIgnoreCase))
+                continue;
+
+            var value = entry.Value;
+            if (value is not null && value.StartsWith(BearerPrefix, StringComparison.OrdinalIgnoreCase))
+                return value[BearerPrefix.Length..];
+        }
+
+        return null;
+    }
+
+    private static bool FixedTimeEquals(string presented, string expected)
+        => CryptographicOperations.FixedTimeEquals(
+            Encoding.UTF8.GetBytes(presented), Encoding.UTF8.GetBytes(expected));
+}

src/ZB.MOM.WW.ScadaBridge.Host/Program.cs

@@ -514,7 +514,11 @@ try
         });
 
         // gRPC server registration
-        builder.Services.AddGrpc();
+        // The interceptor gates ONLY /localdb_sync.v1.LocalDbSync/ — SiteStream calls on
+        // this same pipeline pass through untouched. It is fail-closed: with no
+        // LocalDb:Replication:ApiKey configured, no sync stream is accepted at all.
+        builder.Services.AddGrpc(options =>
+            options.Interceptors.Add<LocalDbSyncAuthInterceptor>());
         builder.Services.AddSingleton<ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteStreamGrpcServer>();
 
         // Existing site service registrations (this is also where LocalDb and its
@@ -540,9 +544,8 @@ try
         // The passive half of LocalDb replication: the peer node dials THIS endpoint.
         // It shares the Kestrel h2c listener the site gRPC server already uses, so no
         // listener or port changes are needed. Mapping it is harmless with no peer
-        // configured — nothing dials it. Inbound auth is the host's job and lands in
-        // Task 8; until then this endpoint is unauthenticated, which is why
-        // replication ships default-OFF.
+        // configured — nothing dials it, and LocalDbSyncAuthInterceptor (registered on
+        // AddGrpc above) rejects anything that tries until an ApiKey is configured.
         app.MapZbLocalDbSync();
 
         // Site-shutdown ordering. ApplicationStopping

src/ZB.MOM.WW.ScadaBridge.Host/SiteServiceRegistration.cs

@@ -100,6 +100,16 @@ public static class SiteServiceRegistration
         services.AddSiteEventLogHealthMetricsBridge(
             sp => () => sp.GetRequiredService<ISiteEventLogger>().FailedWriteCount);
 
+        // LocalDb replication — bridge ISyncStatus onto the site health report. Registered
+        // unconditionally alongside the engine: on a node with no peer this reports
+        // Connected=false with a real 0 backlog, which is the healthy default-OFF state.
+        // OplogBacklog is passed through NULLABLE on purpose — null means the poll failed
+        // or no provider is wired, and flattening it to 0 would render a replication pair
+        // that cannot read its own oplog as perfectly healthy.
+        services.AddLocalDbReplicationHealthBridge(
+            sp => () => sp.GetRequiredService<ISyncStatus>().Connected,
+            sp => () => sp.GetRequiredService<ISyncStatus>().OplogBacklog);
+
         // Audit Log — site-side hot-path writer + telemetry collaborators.
         // The SiteAuditTelemetryActor itself is registered by AkkaHostedService
         // in the site-role block; this call wires every DI dependency it (and