fix(#3517): metrics are not optional, and the endpoint is denied by default

Two review findings, folded together.

mara: a swarm integrated auto deployed forge always has metrics, so the
toggle is gone. The endpoint follows behindGateway instead, which is the
swarm-integrated shape and the condition the protected location lives
under. Serving it without that location would put it on a listener
openFirewall can expose with nothing in front.

argus: /metrics matched no access_control rule, so default_policy
one_factor governed it. That is any authenticated subject, which today
means any operator and tomorrow any agent. My audience argument covered
the Bearer path only; the same endpoint also accepts CookieSession, and a
cookie carries no audience at all, so the audience was never what stood
between a browser session and this data.

The rule is deny rather than a client-scoped allow because the collector's
client does not exist yet. Authelia refuses a subject naming an
unregistered client, and does so in a preStart validator rather than at
build time, so naming one early yields a green nixos-rebuild and dead
swarm SSO on the next restart. Denying until the client is registered
makes publishing the endpoint safe on its own; registering it is a
one-line change from deny to that allow.

Rule order is load-bearing: authelia takes the first match.

nix/host-modules/hive-forge/default.nix

@@ -305,36 +305,6 @@ in
       '';
     };
 
-    metricsEnable = lib.mkOption {
-      type = lib.types.bool;
-      default = false;
-      example = true;
-      description = ''
-        Serve forgejo's prometheus metrics and publish them, behind
-        authelia, at `https://<domain>/metrics`.
-
-        Off by default. Forgejo's metrics carry repository, user and
-        request counts for the whole hive, so turning them on is a
-        disclosure decision an operator should make deliberately rather
-        than inherit from a collector appearing elsewhere in the swarm.
-
-        ::: {.warning}
-        This option is deliberately **one** switch for two changes that
-        must never be made separately. Forgejo serves `/metrics` on its
-        normal HTTP listener, and the gateway vhost proxies `/` to that
-        listener — so enabling the endpoint *without* the protected
-        location would publish it through the existing catch-all, to
-        anyone. The `= /metrics` location added alongside is an exact
-        match and therefore wins over the `/` prefix.
-        :::
-
-        Authentication is the gateway's, not forgejo's own `[metrics]
-        TOKEN`: a scraper presents an audience-scoped authelia token and
-        nginx checks it via `auth_request`, so the swarm keeps one
-        identity system instead of a static bearer per service.
-      '';
-    };
-
     mirrors = lib.mkOption {
       type = lib.types.listOf (
         lib.types.submodule {
@@ -480,10 +450,6 @@ in
       "${cfg.domain}" = (gatewayCfg.lib.tlsFor cfg.domain) // {
         listen = gatewayCfg.lib.listen;
         extraConfig = gatewayCfg.lib.securityHeaders;
-        # ⚠️ The metrics locations merge into THIS attrset, not into the
-        # vhost above it. `//` is shallow: merging at the vhost level
-        # would replace `locations` wholesale and silently drop the `/`
-        # proxy — a config that still renders and still starts.
         locations = {
           "/" = {
             proxyPass = "http://127.0.0.1:${toString cfg.httpPort}/";
@@ -495,14 +461,13 @@ in
               proxy_send_timeout 1h;
             '';
           };
-        }
+
-        // lib.optionalAttrs cfg.metricsEnable {
           # ⚠️ EXACT match, and that is what makes this safe. Forgejo
           # serves `/metrics` on the same listener the `/` prefix above
           # already proxies, so without a more specific location the
-          # endpoint would be public the moment it is enabled.
+          # endpoint would ride that catch-all to anyone. `= /metrics`
-          # `= /metrics` outranks the `/` prefix in nginx, so this
+          # outranks the `/` prefix in nginx, so this location — and its
-          # location — and its auth — is the one that runs.
+          # auth — is the one that runs.
           "= /metrics" = {
             proxyPass = "http://127.0.0.1:${toString cfg.httpPort}/metrics";
             extraConfig = ''
@@ -748,17 +713,20 @@ in
                 DEFAULT_BRANCH = "main";
                 DEFAULT_PRIVATE = "private";
               };
+              # Not an option: a swarm-integrated, auto-deployed forge
+              # always has metrics. Tied to `behindGateway` because that
+              # IS the swarm-integrated shape — it is the condition under
+              # which the protected `= /metrics` location below exists.
+              # Serving the endpoint without that location would put it on
+              # a listener `openFirewall` can expose, with nothing in
+              # front of it.
+              #
               # No `TOKEN` here on purpose. Forgejo can guard this itself
               # with a static bearer, but the swarm authenticates the
-              # scraper at the gateway with an audience-scoped authelia
+              # scraper at the gateway, so a second credential system per
-              # token, so a second credential system per service would buy
+              # service would buy nothing and would be the one that stops
-              # nothing and would be the one that stops getting rotated.
+              # getting rotated.
-              #
+              metrics.ENABLED = cfg.behindGateway;
-              # Reachability is not decided here: forgejo serves this on
-              # its normal listener, which is loopback-only. What publishes
-              # it is the `= /metrics` vhost location, and that location is
-              # what authenticates it.
-              metrics.ENABLED = cfg.metricsEnable;
               # Repo migrations / pull-mirrors fetch from the source
               # URL *inside* Forgejo. hyperhive code is synced from
               # `localhost` (and the host LAN), which Forgejo's

nix/host-modules/swarm-authelia.nix

@@ -42,6 +42,7 @@ let
   hyperhiveDomain = hyperhiveCfg.domain;
   swarmDomain = hyperhiveCfg.swarm.domain;
   uiCfg = hyperhiveCfg.swarm.ui;
+  forgeCfg = hyperhiveCfg.swarm.forge;
 
   # Group an account must hold to reach operator-only surfaces. Named
   # here because this module writes the rule that enforces it and
@@ -1134,11 +1135,41 @@ in
               # the account to disagree silently.
               access_control = {
                 default_policy = "one_factor";
-                rules = lib.optional uiCfg.enable {
+                # ⚠️ ORDER MATTERS — authelia takes the FIRST matching rule.
-                  domain = uiCfg.domain;
+                # The metrics rule is listed first so it cannot be shadowed
-                  subject = [ "group:${operatorGroup}" ];
+                # by a broader domain rule added later.
-                  policy = "one_factor";
+                rules =
-                };
+                  # The forge's metrics endpoint. `deny` is deliberate and
+                  # is the whole protection right now: the endpoint is
+                  # always served (a swarm-integrated forge always has
+                  # metrics), and `default_policy` is `one_factor`, which
+                  # means *any* authenticated subject — every operator
+                  # today, every agent once they hold authelia accounts.
+                  #
+                  # Being reachable by a Bearer token is not sufficient on
+                  # its own: `authn_strategies` on this endpoint also
+                  # accepts `CookieSession`, and a cookie carries no
+                  # audience, so the audience is not what stands between a
+                  # browser session and this data.
+                  #
+                  # The collector gets in by REPLACING this with a
+                  # client-scoped allow (`subject = ["oauth2:client:<id>"]`)
+                  # once such a client is registered. Denying until then is
+                  # what makes publishing the endpoint safe on its own —
+                  # authelia refuses a subject naming a client that is not
+                  # registered, and it does so in a `preStart` validator,
+                  # so naming one early takes the whole SSO service down on
+                  # the next restart rather than failing the build.
+                  lib.optional forgeCfg.behindGateway {
+                    domain = forgeCfg.domain;
+                    resources = [ "^/metrics$" ];
+                    policy = "deny";
+                  }
+                  ++ lib.optional uiCfg.enable {
+                    domain = uiCfg.domain;
+                    subject = [ "group:${operatorGroup}" ];
+                    policy = "one_factor";
+                  };
               };
 
               # The cookie domain is the SWARM's domain, NOT authelia's