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/Http/Controllers/Api/V1/Admin/AdminStatsController.php

@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Controllers\Api\V1\Admin;
+
+use App\Http\Controllers\Controller;
+use App\Models\Event;
+use App\Models\Organisation;
+use App\Models\Person;
+use App\Models\User;
+use Illuminate\Http\JsonResponse;
+
+final class AdminStatsController extends Controller
+{
+    public function index(): JsonResponse
+    {
+        return response()->json([
+            'data' => [
+                'organisations' => [
+                    'total' => Organisation::withoutGlobalScopes()->count(),
+                    'by_billing_status' => Organisation::withoutGlobalScopes()
+                        ->selectRaw('billing_status, COUNT(*) as count')
+                        ->groupBy('billing_status')
+                        ->pluck('count', 'billing_status'),
+                ],
+                'events' => [
+                    'total' => Event::withoutGlobalScopes()->count(),
+                    'by_status' => Event::withoutGlobalScopes()
+                        ->selectRaw('status, COUNT(*) as count')
+                        ->groupBy('status')
+                        ->pluck('count', 'status'),
+                ],
+                'users' => [
+                    'total' => User::count(),
+                    'verified' => User::whereNotNull('email_verified_at')->count(),
+                ],
+                'persons' => [
+                    'total' => Person::withoutGlobalScopes()->count(),
+                ],
+            ],
+        ]);
+    }
+}