docs/sso: name Grafana as the exception, and say it is unconditional

sso.md's "What this doesn't do" list opened with "It doesn't disable
local login" without qualification, which was wrong for Grafana before
the previous commit and is wrong in a different way after it: Grafana now
disables the form for every deployment, not just where authelia happens to
be on the same host.

Names the exception, gives the reason a reader can act on (a default
`admin`/`admin` account on a gateway-published vhost), and states the
consequence plainly — SSO is the only door, so a dead provider locks
everyone out, which is why the OIDC role defaults to Admin.

docs/swarm/sso.md

@@ -242,11 +242,18 @@ page-only check. This was verified that way when the split was introduced.
 
 ## What this doesn't do
 
-- **It doesn't disable local login.** Each service keeps its password
-  database and gains a second door. An identity provider that can take a
-  service offline when it hiccups is worse than one with two ways in.
-  Making authelia the only path is a separate, reversible switch per
-  service (tuwunel's `login_with_password`, forgejo's own setting).
+- **It doesn't disable local login — except on Grafana.** Forgejo and
+  tuwunel keep their password databases and gain a second door: an identity
+  provider that can take a service offline when it hiccups is worse than one
+  with two ways in, and making authelia the only path there is a separate,
+  reversible switch (tuwunel's `login_with_password`, forgejo's own setting).
+  ⚠️ **Grafana is the exception, and not a reversible one.** Its module sets
+  `auth.disable_login_form` unconditionally, because Grafana ships an
+  `admin`/`admin` account and its vhost is on the public gateway — so a
+  password box there is a way in whatever the deployment. SSO is the only
+  door, and a dead provider locks everyone out, the built-in admin
+  included. That's why `swarm.grafana.oidc.role` defaults to `Admin`; see
+  [`services.md`](services.md#metrics-victoriametrics--grafana).
 - **It doesn't provision users.** Agents are created and destroyed
   continuously, so the subject set belongs to a program rather than to a
   config file; today that program is `swarmctl`.

nix/host-modules/swarm-grafana.nix

@@ -701,10 +701,17 @@ in
               # whole host fails to build. The unit below generates it.
               security.secret_key = "$__file{${secretKeyPath}}";
 
-              # No local password path at all when SSO is configured. This
-              # is not tidiness: Grafana ships an `admin`/`admin` account,
-              # and this vhost is on the public gateway.
-              auth.disable_login_form = ssoLocal;
+              # No local password path, ever. This is not tidiness: Grafana
+              # ships an `admin`/`admin` account and this vhost is on the
+              # public gateway, so SSO is the only way in — not merely the
+              # preferred one.
+              #
+              # Deliberately NOT `ssoLocal`: whether a password box sits on a
+              # public login page cannot depend on which host happens to run
+              # authelia. The OIDC block below is still gated on locality;
+              # making that follow the same swarm-wide question is a larger
+              # change tracked on its own.
+              auth.disable_login_form = true;
             }
             // lib.optionalAttrs ssoLocal {
               "auth.generic_oauth" = {

nix/module-eval.nix

@@ -655,6 +655,16 @@ let
         rule: lib.hasInfix "/run/test-grafana-sock" rule
       ) grafanaOldPath.systemd.tmpfiles.rules;
     }
+    {
+      # This fixture enables grafana and NOT authelia, which is the shape the
+      # login form used to stay enabled in: the toggle read "both services are
+      # on this host" rather than "grafana requires SSO". Grafana ships an
+      # `admin`/`admin` account and its vhost is on the public gateway, so a
+      # password box there is a way in whatever the topology.
+      name = "grafana disables its local login form even where authelia is not on this host";
+      ok =
+        grafanaOldPath.containers.swarm-grafana.config.services.grafana.settings.auth.disable_login_form;
+    }
     {
       # Reads the daemon's rendered unit, not the options: the queue address
       # arrives as an env var whose whole attrset is guarded on `natsUrl`, and