feat(grpc): PSK-authenticate the site gRPC control plane; drop the vestigial management receptionist registration

Phase 0 of the ClusterClient→gRPC migration
(docs/plans/2026-07-22-clusterclient-to-grpc-plan.md). Standalone hardening: it
closes a gap that exists today and is a precondition for moving command/control
onto gRPC in later phases.

T0.1 — delete the ManagementActor ClusterClientReceptionist registration.
It was built for an out-of-cluster CLI that was never written: the shipped CLI
speaks HTTP Basic to /management, which asks the actor in-process through
ManagementActorHolder. Nothing in the repo ever sent to /user/management. The
actor still runs there; only the cross-boundary advertisement is gone. Six
documents claimed the CLI used ClusterClient — including the CLI's own README
"Architecture Notes" — and are corrected here rather than left to rot.

T0.2 — record, do not port, the dead integration-routing path.
IntegrationCallRequest is unwired at BOTH ends: RouteIntegrationCallAsync has
zero callers anywhere, and RegisterLocalHandler(Integration, …) appears only in
a test, so production always answers "Integration handler not available". It is
excluded from the gRPC contract (28 of 29 commands migrate) rather than
enshrined on an additive-only wire format, and deleting it during a
transport migration would mix a behavioural change into a change whose whole
value is that behaviour is identical. See
docs/known-issues/2026-07-22-integration-call-routing-is-dead-code.md.

T0.3 — preshared-key authentication on SiteStreamService.
The service shipped with no auth at all: plaintext h2c, no interceptor, so
anything that could reach a site node's :8083 could open a live data stream or
read audit rows back via PullAuditEvents/PullSiteCalls. ControlPlaneAuthInterceptor
now gates /sitestream.SiteStreamService/ — modeled on LocalDbSyncAuthInterceptor
(constant-time compare, fail-closed, PermissionDenied) but gating a SET of
service prefixes so phases 1A/1B add services rather than interceptors. LocalDb
sync keeps its own separate key: it authenticates the pair partner, not central,
and collapsing the two would make a site's central-facing key also admit writes
into its database.

Keys are per site (SB-GRPC-PSK-<siteId>), never fleet-wide, so a compromised
site yields only its own. Central attaches them through ControlPlaneCredentials,
which binds CallCredentials to the channel — covering unary and streaming
uniformly, and letting the key resolve asynchronously, which a client
interceptor could not do without blocking. All three central→site channel
creation sites go through it (SiteStreamGrpcClient and both audit pull invokers);
the pull invokers' channel caches are re-keyed by (site, endpoint) because
credentials are per-site and bound to the channel.

Two decisions beyond the plan:

  * StartupValidator now requires GrpcPsk on Site nodes. The plan specified only
    the runtime gate, but fail-closed with no boot check produces a node that
    joins, answers heartbeats and reports healthy while refusing every stream,
    audit pull and telemetry ingest — silent and total. Same reasoning as the
    existing inbound API-key pepper rule.

  * Added Communication:SitePsks as a central-side key map. The plan assumed
    central would read the store, seeded via a dev KEK; the docker rig
    deliberately boots with no master key, so store-only resolution would leave
    it unable to dial its own sites. The store stays primary — it is the only
    source that can serve a site added at runtime — with the map covering
    key-less hosts and one-off pins. Neither source falling back to
    "unauthenticated" is the invariant.

T0.4 — dev keys on both rigs and tests.
34 tests. The seven that matter most exercise a real in-process gRPC stack over
TestServer: the unit tests on either side of the wire would both stay green if
the halves disagreed, and gRPC refuses call credentials on a plaintext channel
by default — the UnsafeUseInsecureChannelCallCredentials opt-in is only provable
by making a real call. They confirm correct key passes on unary AND streaming,
wrong key and no-credentials both get PermissionDenied, and an unresolvable key
fails the call with nothing reaching the service.

OPERATIONAL: a site node upgraded to this build without a key will not boot.
That includes the gitignored deploy/wonder-app-vd03/ overlay.

src/ZB.MOM.WW.ScadaBridge.Host/Actors/AkkaHostedService.cs

@@ -450,16 +450,22 @@ akka {{
         siteAlarmLiveCache?.SetActorSystem(_actorSystem!);
 
 
-        // Management Service — accessible via ClusterClient
+        // Management Service — reached IN-PROCESS only, via ManagementActorHolder.
+        //
+        // This actor used to be registered with the ClusterClientReceptionist as well, for
+        // an out-of-cluster CLI that was never built (REQ-HOST-6a). The shipped CLI speaks
+        // HTTP Basic to /management, which asks this actor through the holder below
+        // (ManagementEndpoints), so the registration had no sender anywhere in the repo —
+        // it only advertised a management surface across the cluster-client boundary for
+        // free. Removed 2026-07-22 (ClusterClient→gRPC migration, T0.1).
         var mgmtLogger = _serviceProvider.GetRequiredService<ILoggerFactory>()
             .CreateLogger<ZB.MOM.WW.ScadaBridge.ManagementService.ManagementActor>();
         var mgmtActor = _actorSystem!.ActorOf(
             Props.Create(() => new ZB.MOM.WW.ScadaBridge.ManagementService.ManagementActor(_serviceProvider, mgmtLogger)),
             "management");
-        ClusterClientReceptionist.Get(_actorSystem).RegisterService(mgmtActor);
         var mgmtHolder = _serviceProvider.GetRequiredService<ZB.MOM.WW.ScadaBridge.ManagementService.ManagementActorHolder>();
         mgmtHolder.ActorRef = mgmtActor;
-        _logger.LogInformation("ManagementActor registered with ClusterClientReceptionist");
+        _logger.LogInformation("ManagementActor started at /user/management (in-process access via ManagementActorHolder)");
 
         // Notification Outbox — cluster singleton so exactly one node owns ingest,
         // the dispatch sweep and the purge loop. Central actors run on the base