feat(3167): authelia gates the swarm UI on an operators group

default_policy = one_factor means 'any authenticated user', which is
authentication and not authorisation. The swarm UI is operator-only and
agents are getting authelia accounts of their own, so a session alone
would be enough to open it the day that lands - the vhost's auth_request
would be a check nobody fails.

Adds an access_control rule for the UI's domain requiring
subject = group:operators, present only when the UI is enabled. The
group name is a constant beside the rule: it is also what an operator
types into 'swarmctl user add --group', and a configurable name is one
more way for the rule and the account to disagree silently.

nix/host-modules/swarm-authelia.nix

@@ -37,6 +37,13 @@ let
   hyperhiveCfg = config.services.hyperhive;
   hyperhiveDomain = hyperhiveCfg.domain;
   swarmDomain = hyperhiveCfg.swarm.domain;
+  uiCfg = hyperhiveCfg.swarm.ui;
+
+  # Group an account must hold to reach operator-only surfaces. Named
+  # here because this module writes the rule that enforces it and
+  # `swarmctl user add --group <this>` is what grants it — the two must
+  # agree, and one constant is how they stay agreeing.
+  operatorGroup = "operators";
 
   # Upstream's `services.authelia.instances.<name>` derives the unit,
   # user, group and StateDirectory from the instance name
@@ -536,7 +543,26 @@ in
 
               authentication_backend.file.path = cfg.usersFile;
 
-              access_control.default_policy = "one_factor";
+              # ⚠️ `one_factor` as the DEFAULT means "any authenticated
+              # user", which is authentication, not authorisation. The
+              # swarm UI is operator-only, and agents are getting
+              # authelia accounts of their own — so the day that lands,
+              # a session alone would be enough to open it. The rule
+              # below is what makes the distinction real; without it
+              # the vhost's `auth_request` is a check nobody fails.
+              #
+              # The group is a constant rather than an option: it is the
+              # value an operator types into `swarmctl user add --group`,
+              # and a configurable name is one more way for the rule and
+              # the account to disagree silently.
+              access_control = {
+                default_policy = "one_factor";
+                rules = lib.optional uiCfg.enable {
+                  domain = uiCfg.domain;
+                  subject = [ "group:${operatorGroup}" ];
+                  policy = "one_factor";
+                };
+              };
 
               # The cookie domain is the SWARM's domain, NOT authelia's
               # own host: the session cookie has to be sent to the apps