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/app/Enums/BillingStatus.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Enums;
+
+enum BillingStatus: string
+{
+    case TRIAL = 'trial';
+    case ACTIVE = 'active';
+    case SUSPENDED = 'suspended';
+    case CANCELLED = 'cancelled';
+
+    public function label(): string
+    {
+        return match ($this) {
+            self::TRIAL => 'Trial',
+            self::ACTIVE => 'Active',
+            self::SUSPENDED => 'Suspended',
+            self::CANCELLED => 'Cancelled',
+        };
+    }
+}