feat: add HTTP Management API, migrate CLI from Akka ClusterClient to HTTP

Replace the CLI's Akka.NET ClusterClient transport with a simple HTTP client
targeting a new POST /management endpoint on the Central Host. The endpoint
handles Basic Auth, LDAP authentication, role resolution, and ManagementActor
dispatch in a single round-trip — eliminating the CLI's Akka, LDAP, and
Security dependencies.

Also fixes DCL ReSubscribeAll losing subscriptions on repeated reconnect by
deriving the tag list from _subscriptionsByInstance instead of _subscriptionIds.

src/ScadaLink.ManagementService/ManagementActorHolder.cs

@@ -0,0 +1,8 @@
+using Akka.Actor;
+
+namespace ScadaLink.ManagementService;
+
+public class ManagementActorHolder
+{
+    public IActorRef? ActorRef { get; set; }
+}