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/Requests/Admin/AdminUpdateOrganisationRequest.php

@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Requests\Admin;
+
+use App\Enums\BillingStatus;
+use Illuminate\Foundation\Http\FormRequest;
+use Illuminate\Validation\Rule;
+use Illuminate\Validation\Rules\Enum;
+
+final class AdminUpdateOrganisationRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /** @return array<string, mixed> */
+    public function rules(): array
+    {
+        return [
+            'name' => ['sometimes', 'string', 'max:255'],
+            'slug' => [
+                'sometimes', 'string', 'max:255', 'regex:/^[a-z0-9-]+$/',
+                Rule::unique('organisations', 'slug')->ignore($this->route('organisation')),
+            ],
+            'billing_status' => ['sometimes', new Enum(BillingStatus::class)],
+            'settings' => ['nullable', 'array'],
+        ];
+    }
+}