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/Resources/Admin/AdminUserResource.php

@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Resources\Admin;
+
+use Illuminate\Http\Request;
+use Illuminate\Http\Resources\Json\JsonResource;
+
+final class AdminUserResource extends JsonResource
+{
+    public function toArray(Request $request): array
+    {
+        return [
+            'id' => $this->id,
+            'first_name' => $this->first_name,
+            'last_name' => $this->last_name,
+            'full_name' => $this->full_name,
+            'email' => $this->email,
+            'avatar' => $this->avatar,
+            'timezone' => $this->timezone,
+            'locale' => $this->locale,
+            'email_verified_at' => $this->email_verified_at?->toIso8601String(),
+            'created_at' => $this->created_at->toIso8601String(),
+            'roles' => $this->getRoleNames()->values()->all(),
+            'is_super_admin' => $this->hasRole('super_admin'),
+            'organisations' => $this->whenLoaded('organisations', fn () =>
+                $this->organisations->map(fn ($org) => [
+                    'id' => $org->id,
+                    'name' => $org->name,
+                    'slug' => $org->slug,
+                    'role' => $org->pivot->role,
+                ])
+            ),
+        ];
+    }
+}