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.

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/StartupValidatorTests.cs

@@ -38,6 +38,9 @@ public class StartupValidatorTests
         ["ScadaBridge:Database:SiteDbPath"] = "./data/scadabridge.db",
         ["ScadaBridge:Cluster:SeedNodes:0"] = "akka.tcp://scadabridge@site-a-node1:8082",
         ["ScadaBridge:Cluster:SeedNodes:1"] = "akka.tcp://scadabridge@site-a-node2:8082",
+        // T0.3: the gRPC control plane is fail-closed, so a Site node without a preshared
+        // key serves nothing while still looking healthy. Required at boot for that reason.
+        ["ScadaBridge:Communication:GrpcPsk"] = "test-site-control-plane-key",
     };
 
     [Fact]
@@ -56,6 +59,44 @@ public class StartupValidatorTests
         Assert.Null(ex);
     }
 
+    [Fact]
+    public void SiteWithoutGrpcPsk_FailsValidation()
+    {
+        // The failure this prevents is silent: ControlPlaneAuthInterceptor refuses every
+        // SiteStream call without a key, so the node boots, joins its pair, reports healthy —
+        // and serves no live streams, no audit pulls and no cached-telemetry ingest. Central
+        // sees a site that is up and answering heartbeats but never sends anything.
+        var values = ValidSiteConfig();
+        values.Remove("ScadaBridge:Communication:GrpcPsk");
+        var config = BuildConfig(values);
+
+        var ex = Assert.Throws<InvalidOperationException>(() => StartupValidator.Validate(config));
+        Assert.Contains("GrpcPsk", ex.Message);
+    }
+
+    [Fact]
+    public void SiteWithBlankGrpcPsk_FailsValidation()
+    {
+        // Whitespace is not a key. An empty-string value would otherwise satisfy a
+        // key-present check while leaving the interceptor in its fail-closed state.
+        var values = ValidSiteConfig();
+        values["ScadaBridge:Communication:GrpcPsk"] = "   ";
+        var config = BuildConfig(values);
+
+        var ex = Assert.Throws<InvalidOperationException>(() => StartupValidator.Validate(config));
+        Assert.Contains("GrpcPsk", ex.Message);
+    }
+
+    [Fact]
+    public void CentralWithoutGrpcPsk_PassesValidation()
+    {
+        // Central holds one key PER SITE (SitePsks / the secret store), not a single key of
+        // its own, so this setting is meaningless there and must not be required.
+        var config = BuildConfig(ValidCentralConfig());
+
+        Assert.Null(Record.Exception(() => StartupValidator.Validate(config)));
+    }
+
     [Fact]
     public void MissingRole_FailsValidation()
     {