fix(admin): authenticate SignalR hub clients with a bearer-token scheme

The Admin-003 fix gated every SignalR hub with [Authorize], but the server-side
Blazor HubConnection clients had no way to authenticate: the browser's HttpOnly
auth cookie is not reachable from the interactive circuit, so every hub negotiate
returned 401 and the Admin live-update feature was non-functional app-wide
(silently degraded on Hosts/ScriptLog, fatal on the cluster pages).

Introduce a token-based hub auth path:
- HubTokenService mints/validates short-lived tokens using ASP.NET Core Data
  Protection (the same primitive that protects the auth cookie — no signing-key
  management, no new packages). Tokens carry the user's name + roles.
- HubTokenAuthenticationHandler is a custom "HubToken" auth scheme that reads the
  token from the Authorization: Bearer header (negotiate) or the access_token
  query parameter (WebSocket upgrade).
- The "HubClients" authorization policy runs both the cookie and HubToken
  schemes; the hub endpoints use RequireAuthorization("HubClients").
- AdminHubConnectionFactory builds hub connections with an AccessTokenProvider
  that mints a fresh token for the circuit's authenticated user on every
  (re)connect. All six hub-consuming pages now resolve connections through it.

Hub negotiate now returns 200 and the WebSocket upgrades (101); live updates
work. The best-effort try/catch guards added previously are kept as defence.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Server/ZB.MOM.WW.OtOpcUa.Admin/Components/Pages/RoleGrants.razor

@@ -12,6 +12,7 @@
 @inject ClusterService ClusterSvc
 @inject AclChangeNotifier Notifier
 @inject NavigationManager Nav
+@inject AdminHubConnectionFactory HubFactory
 @implements IAsyncDisposable
 
 <h1 class="page-title">LDAP group → Admin role grants</h1>
@@ -171,10 +172,7 @@ else
     protected override async Task OnAfterRenderAsync(bool firstRender)
     {
         if (!firstRender || _hub is not null) return;
-        _hub = new HubConnectionBuilder()
-            .WithUrl(Nav.ToAbsoluteUri("/hubs/fleet"))
-            .WithAutomaticReconnect()
-            .Build();
+        _hub = HubFactory.Create("/hubs/fleet");
         _hub.On<RoleGrantsChangedMessage>("RoleGrantsChanged", async _ =>
         {
             await ReloadAsync();