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.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