swarm-grafana: provision a queue (nats) dashboard

Split out of the combined swarm-services board, which mara declined as
multiple dashboards in one trench coat. Its 17 panels partition exactly
by subject -- 3 services / 5 metrics store / 5 queue / 4 log store, with
no panel belonging to two -- so this is a re-cut of verified panels, not
new authoring.

Seven panels: connections, subscriptions, slow consumers and JetStream
API errors as stats, messages over the range, plus pending bytes (the
leading indicator for a slow consumer) and JetStream store size against
its configured limit (the only capacity question the queue has).

Connections is red at zero, unlike every other count here: three things
hold a connection at all times, so an empty count means the queue is
refusing everyone -- and a callout denial reaches a client as a timeout
rather than an error, which makes this often the first visible sign.

nix/host-modules/swarm-grafana.nix

@@ -61,6 +61,7 @@ let
         "agents.json"
         "authelia.json"
         "claude-usage.json"
+        "queue.json"
       ]
   );