feat: platform admin backend — controllers, services, routes, tests

Add cross-organisation admin API endpoints behind role:super_admin middleware:
- AdminOrganisationController: CRUD with search, filter, billing_status management
- AdminUserController: user management with role assignment across orgs
- AdminStatsController: platform-wide aggregate statistics
- AdminActivityLogController: filterable activity log viewer
- AdminImpersonationController + ImpersonationService: user impersonation with
  token-based session management and activity logging
- BillingStatus enum, form requests, API resources, 23 feature tests

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/bootstrap/app.php

@@ -34,6 +34,7 @@ return Application::configure(basePath: dirname(__DIR__))
 
         $middleware->alias([
             'portal.token' => \App\Http\Middleware\PortalTokenMiddleware::class,
+            'role' => \Spatie\Permission\Middleware\RoleMiddleware::class,
         ]);
     })
     ->withExceptions(function (Exceptions $exceptions): void {