feat(host+rig): give central a dedicated Prometheus scrape port (register row 56)

The row read 'central nodes expose no Prometheus endpoint'. That is not quite
what was wrong, and the difference matters. Central already maps /metrics
(Program.cs central branch) and telemetry is registered for both roles through
the shared BindSharedOptions, so the endpoint rides endpoint routing and has
always been served on central's web port (:5000 on the rig, anonymous — there
is no FallbackPolicy). What central lacked was the site branch's dedicated
MetricsPort, which is the port the established live-scrape recipe probes:

    docker exec <node> bash -c 'exec 3<>/dev/tcp/localhost/8084 ...'

So a probe of central on 8084 failed and central-only series (KPI/outbox
counters, the bundle-import session gauge + its new rejection counter,
alarm-cache gauges) read as unscrapeable while they were in fact being
exported on a different port.

Central now binds a dedicated HTTP/1.1 listener on ScadaBridge:Node:MetricsPort,
so one probe recipe covers all eight rig nodes. HTTP/1.1-capable because a
standard Prometheus scraper cannot negotiate h2c, which is why this cannot
share the gRPC port.

Read as a PRESENCE check, not GetValue's defaulted read. NodeOptions defaults
MetricsPort to 8084, so a defaulted read would make every existing central node
start binding a new port on upgrade — and a port already in use is a hard boot
failure, not a degraded one. Central gains the listener only where an operator
asked for it; site nodes are unchanged and still bind theirs unconditionally.
The bind is also skipped when the port would collide with the gRPC port or an
ASPNETCORE_URLS port, since Kestrel throws on a duplicate ListenAnyIP and
/metrics is reachable on those ports anyway.

Config only — no host port mapping added, because the probe runs inside the
container. Applied to both docker/ centrals and both docker-env2/ centrals.

FILE EDITS ONLY: no docker command was run, and deploy.sh was not invoked.

docker/central-node-a/appsettings.Central.json

@@ -4,7 +4,9 @@
       "Role": "Central",
       "NodeName": "central-a",
       "NodeHostname": "scadabridge-central-a",
-      "RemotingPort": 8081
+      "RemotingPort": 8081,
+      "_metricsPort": "Register row 56. Central maps /metrics on every listener it binds, so it was always reachable on the web port (:5000) — but not on 8084, which is the port the live-scrape recipe probes (docker exec ... </dev/tcp/localhost/8084). Declaring it here binds the same dedicated HTTP/1.1 scrape listener the six site nodes use, so one probe recipe works on all eight nodes and central-only series (KPI/outbox counters, the import-session gauge + rejection counter, alarm-cache gauges) are scrapeable live. Central reads this key ONLY when present — unlike a site node, which always binds 8084 — so an existing central without it is unchanged. No host port mapping is needed: the probe runs inside the container.",
+      "MetricsPort": 8084
     },
     "Cluster": {
       "SeedNodes": [