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/tests/Feature/Api/V1/Admin/AdminStatsControllerTest.php

@@ -0,0 +1,66 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature\Api\V1\Admin;
+
+use App\Models\Event;
+use App\Models\Organisation;
+use App\Models\User;
+use Database\Seeders\RoleSeeder;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Laravel\Sanctum\Sanctum;
+use Tests\TestCase;
+
+class AdminStatsControllerTest extends TestCase
+{
+    use RefreshDatabase;
+
+    private User $superAdmin;
+    private User $regularUser;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+        $this->seed(RoleSeeder::class);
+
+        $this->superAdmin = User::factory()->create();
+        $this->superAdmin->assignRole('super_admin');
+
+        $this->regularUser = User::factory()->create();
+    }
+
+    public function test_returns_aggregate_counts(): void
+    {
+        $org = Organisation::factory()->create(['billing_status' => 'active']);
+        Event::factory()->count(2)->create([
+            'organisation_id' => $org->id,
+            'status' => 'draft',
+        ]);
+
+        Sanctum::actingAs($this->superAdmin);
+
+        $response = $this->getJson('/api/v1/admin/stats');
+
+        $response->assertOk();
+        $response->assertJsonStructure([
+            'data' => [
+                'organisations' => ['total', 'by_billing_status'],
+                'events' => ['total', 'by_status'],
+                'users' => ['total', 'verified'],
+                'persons' => ['total'],
+            ],
+        ]);
+        $this->assertGreaterThanOrEqual(1, $response->json('data.organisations.total'));
+        $this->assertGreaterThanOrEqual(2, $response->json('data.events.total'));
+    }
+
+    public function test_denied_for_non_super_admin(): void
+    {
+        Sanctum::actingAs($this->regularUser);
+
+        $response = $this->getJson('/api/v1/admin/stats');
+
+        $response->assertForbidden();
+    }
+}