fix(high-severity): close 9 of 10 open High findings across 8 modules

Comm-016: delete dead HandleConnectionStateChanged + _debugSubscriptions /
_inProgressDeployments tracking + ConnectionStateChanged message record.
Disconnect detection is owned by the transport layers (gRPC keepalive PING
~25s; Ask-timeout at CommunicationService). Updates the
Component-Communication.md design doc to make that explicit.

SnF-018: NotificationForwarder.DeliverAsync now discards a corrupt buffered
payload (Warning log + return true) instead of returning false and parking
the row — honoring the design's "notifications do not park" invariant.

DM-018: reconciliation no longer force-sets Enabled, preserving an
intentional Disabled state after central failover.

ESG-018: DeliverBufferedAsync (both ExternalSystemClient + DatabaseGateway)
catches JsonException and returns false, turning a corrupt buffered row
into a parked operation instead of a retry-forever poison message.

InboundAPI-022: register ActiveNodeGate as IActiveNodeGate in the Central
DI branch so standby-node gating is actually wired up in production.

NS-019: remove orphaned NotificationDeliveryService /
INotificationDeliveryService / NotificationResult; central notification
delivery now lives entirely in NotificationOutbox.

SEL-016: normalise From/To filters to UTC before ISO-string compare so
non-UTC DateTimeOffset clients no longer get spuriously excluded events.

TE-017: include Description on attributes/alarms and a HashableConnections
projection (protocol, endpoint JSON, failover count) in the revision hash
and DiffService; staleness detection now catches description-only and
connection-endpoint edits.

Transport-001 and Transport-002 (also High) remain Open — they're being
handled in a follow-up batch because both touch BundleImporter.cs and
must serialise.

src/ScadaLink.Host/Program.cs

@@ -120,6 +120,16 @@ try
         builder.Services.AddSingleton<AkkaHostedService>();
         builder.Services.AddHostedService(sp => sp.GetRequiredService<AkkaHostedService>());
 
+        // InboundAPI-022: register the production IActiveNodeGate implementation so
+        // standby-node gating is actually enforced (the InboundApiEndpointFilter
+        // consults IActiveNodeGate and defaults to "allow" when none is registered,
+        // which leaves the design's "central cluster only (active node)" guarantee
+        // unenforced in deployed binaries). The gate is backed by the same Akka
+        // cluster-leadership check as ActiveNodeHealthCheck above, so the inbound
+        // API and the /health/active endpoint Traefik routes against agree on
+        // which node is active.
+        builder.Services.AddSingleton<ScadaLink.InboundAPI.IActiveNodeGate, ActiveNodeGate>();
+
         // Cluster node status provider scoped to the Central role — feeds the
         // CentralHealthReportLoop so the central cluster appears on /monitoring/health.
         builder.Services.AddSingleton<IClusterNodeProvider>(sp =>