feat(swarm-grafana): provision an authelia dashboard

The swarm-services board was mostly authelia, so it becomes its own page
and is trimmed out of that one (mara, #3494). Eleven panels: uptime,
authentications and failures, authorization decisions, requests and
verdicts by status code, request and OIDC latency, and the three Go
process signals.

OIDC latency gets its own panel rather than being folded into the
general one because authelia keeps a separate duration family for it,
and every machine-to-machine credential in this swarm is minted through
those endpoints -- averaging the two produces a number describing
neither.

Counters are counted over the dashboard range rather than rated. At this
volume a rate window contains no requests, so rate() returns zero and
draws a flat line, which is indistinguishable from a broken query; the
quantile panels are worse, because a quantile over all-zero buckets is
NaN and renders empty rather than zero.

nix/host-modules/swarm-grafana.nix

@@ -53,6 +53,7 @@ let
       })
       [
         "agents.json"
+        "authelia.json"
         "claude-usage.json"
       ]
   );