feat(localdb): wire 2-node replication for the site database, default OFF

Task 7 of the LocalDb Phase 1 adoption plan.

AddZbLocalDbReplication registers the engine; MapZbLocalDbSync exposes the
passive endpoint the peer node dials. Both are unconditional but INERT by
default: with no LocalDb:Replication:PeerAddress the initiating
SyncBackgroundService starts and immediately idles, and nothing dials the
endpoint. A site pair replicates only once an operator sets a peer on BOTH
nodes. The endpoint shares the Kestrel h2c listener the site gRPC server already
uses, so there are no listener or port changes.

Deviation from the plan: it put AddZbLocalDbReplication in Program.cs. Doing so
would have left it untested - the composition-root tests build
SiteServiceRegistration's graph, not Program.cs - so a registration that silently
went missing would still show green. It now sits next to AddZbLocalDb in
SiteServiceRegistration, where the DI-graph tests actually cover it.
MapZbLocalDbSync stays in Program.cs because it needs the WebApplication.

Two pins added to SiteLocalDbWiringTests, which configures no peer:
  - the engine resolves and is idle (not connected, no peer, never synced) -
    default-off means inert, not unregistered;
  - OplogBacklog is 0, not null. It is deliberately nullable so a failed poll
    reads as "unknown" instead of a healthy zero; a real 0 proves the provider is
    wired to the oplog, which Task 9's health signal depends on.

Inbound auth on the sync endpoint is Task 8. Until that lands the endpoint is
unauthenticated - which is precisely why replication ships default-OFF and no
config in this repo sets a peer.

Verified: build 0 warnings; Host 296/296, SiteEventLogging 70/70,
SiteRuntime 530/530, Commons 684/684, Communication 312/312.

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

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

@@ -7,6 +7,7 @@ using ZB.MOM.WW.Auth.AspNetCore;
 using ZB.MOM.WW.Health;
 using ZB.MOM.WW.Health.Akka;
 using ZB.MOM.WW.Health.EntityFrameworkCore;
+using ZB.MOM.WW.LocalDb.Replication;
 using ZB.MOM.WW.ScadaBridge.AuditLog;
 using ZB.MOM.WW.ScadaBridge.CentralUI;
 using ZB.MOM.WW.ScadaBridge.ClusterInfrastructure;
@@ -516,7 +517,8 @@ try
         builder.Services.AddGrpc();
         builder.Services.AddSingleton<ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteStreamGrpcServer>();
 
-        // Existing site service registrations
+        // Existing site service registrations (this is also where LocalDb and its
+        // replication engine are registered — see SiteServiceRegistration)
         SiteServiceRegistration.Configure(builder.Services, builder.Configuration);
 
         var app = builder.Build();
@@ -535,6 +537,14 @@ try
         // Map gRPC service — resolves the singleton SiteStreamGrpcServer from DI
         app.MapGrpcService<ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteStreamGrpcServer>();
 
+        // 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.
+        app.MapZbLocalDbSync();
+
         // Site-shutdown ordering. ApplicationStopping
         // fires BEFORE IHostedService.StopAsync runs, so the gRPC server
         // refuses new streams (Unavailable) and cancels every active stream