nix: make swarm.authelia.url non-nullable, trim its docs

Review response on #4620: not having SSO is not a supported
deployment, so the type should not permit it, and the docs paragraph
explaining why SSO is always present is redundant once the type says
so.

- swarm.authelia.url drops types.nullOr.
- Every consumer's null-arm is gone: two option defaults
  (swarm-controller's and swarm's own statusPublish.tokenEndpoint)
  that produced an empty/null placeholder when the URL was null now
  unconditionally compute the real derived URL. Five now-dead
  "assertion = ... != null" guards (swarm-authelia's bridge,
  swarm-grafana, swarm-otel, swarm-nats, hive-forge, hive-matrix) are
  removed as unreachable — in every case the same URL was already
  interpolated unconditionally a few lines below the guard.
- grafanaNoSso, the module-eval fixture whose sole purpose was
  exercising the now-unsupported no-IdP refusal, is removed along
  with its dedicated test case; swarm.authelia.url = null is a type
  error now, not a value that reaches that assertion.
- docs/swarm/services.md: cut the clause about setting the option to
  null and the sentence explaining why the URL is co-location-
  independent — both redundant now that the type enforces it.

nix/host-modules/swarm-nats.nix

@@ -511,18 +511,6 @@ in
           to have this host mint all four.
         '';
       }
-      {
-        assertion = autheliaUrl != null;
-        message = ''
-          services.hyperhive.deploy.nats.enable requires
-          services.hyperhive.swarm.authelia.url — the queue authenticates
-          clients by validating tokens that authelia issued.
-
-          It defaults to this host's own instance only when this host
-          runs authelia. A hive that federates with a swarm sets it
-          explicitly to wherever that provider lives.
-        '';
-      }
     ];
 
     # One declaration, two readers. The queue knows which client id it