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/StartupValidator.cs

@@ -126,6 +126,23 @@ public static class StartupValidator
                 p.Require("ScadaBridge:Node:MetricsPort", _ => metricsPort != port, "must differ from RemotingPort");
                 p.Require("ScadaBridge:Node:MetricsPort", _ => metricsPort != grpcPort, "must differ from GrpcPort");
 
+                // The gRPC control-plane preshared key. Same argument as the inbound
+                // API-key pepper above: without it the node boots and looks healthy, but
+                // ControlPlaneAuthInterceptor is fail-closed, so every SiteStream call —
+                // live subscriptions, audit pulls, cached-telemetry ingest — is refused
+                // with PermissionDenied. A silent, total loss of the site's central-facing
+                // surface is far worse than a loud boot failure, so require it here.
+                // Central holds the matching value per site in its secret store as
+                // SB-GRPC-PSK-{SiteId}; production supplies this one as
+                // ${secret:SB-GRPC-PSK-<siteId>}, expanded before the host is built.
+                p.Require("ScadaBridge:Communication:GrpcPsk",
+                    value => !string.IsNullOrWhiteSpace(value),
+                    "is required for Site nodes: it is the preshared key the gRPC control "
+                    + "plane authenticates with, and the interceptor is fail-closed, so an "
+                    + "unset key refuses every SiteStream call. Set the same value here (in "
+                    + "production as ${secret:SB-GRPC-PSK-<siteId>}) and under the secret "
+                    + "name SB-GRPC-PSK-<siteId> in central's secret store");
+
                 // ScadaBridge:Database:SiteDbPath was required here until LocalDb
                 // Phase 2. The site's tables now live in the consolidated LocalDb
                 // database (LocalDb:Path, which SiteServiceRegistration requires),