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.nix

@@ -500,8 +500,8 @@ in
   options.services.hyperhive.swarm.statusPublish = {
     tokenEndpoint = lib.mkOption {
       type = lib.types.nullOr lib.types.str;
-      default = if swarmCfg.authelia.url != null then "${swarmCfg.authelia.url}/api/oidc/token" else null;
-      defaultText = lib.literalExpression ''"''${swarm.authelia.url}/api/oidc/token" when that URL is set, else null'';
+      default = "${swarmCfg.authelia.url}/api/oidc/token";
+      defaultText = lib.literalExpression ''"''${swarm.authelia.url}/api/oidc/token"'';
       example = "https://auth.example.com/api/oidc/token";
       description = ''
         The swarm IdP's OAuth2 token endpoint. This hive mints a