feat(infra): add Traefik load balancer with active node health check for central cluster failover

Add ActiveNodeHealthCheck that returns 200 only on the Akka.NET cluster
leader, enabling Traefik to route traffic to the active central node and
automatically fail over when the leader changes. Also fixes AkkaClusterHealthCheck
to resolve ActorSystem from AkkaHostedService (was always null via DI).

docker/traefik/dynamic.yml

@@ -0,0 +1,18 @@
+http:
+  routers:
+    central:
+      rule: "PathPrefix(`/`)"
+      service: central
+      entryPoints:
+        - web
+
+  services:
+    central:
+      loadBalancer:
+        healthCheck:
+          path: /health/active
+          interval: 5s
+          timeout: 3s
+        servers:
+          - url: "http://scadalink-central-a:5000"
+          - url: "http://scadalink-central-b:5000"

docker/traefik/traefik.yml

@@ -0,0 +1,11 @@
+entryPoints:
+  web:
+    address: ":80"
+
+api:
+  dashboard: true
+  insecure: true
+
+providers:
+  file:
+    filename: /etc/traefik/dynamic.yml