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/AdminActivityLogController.php

@@ -0,0 +1,63 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Controllers\Api\V1\Admin;
+
+use App\Http\Controllers\Controller;
+use Illuminate\Http\JsonResponse;
+use Spatie\Activitylog\Models\Activity;
+
+final class AdminActivityLogController extends Controller
+{
+    public function index(): JsonResponse
+    {
+        $query = Activity::query()->with('causer')->latest();
+
+        if ($causerId = request('causer_id')) {
+            $query->where('causer_id', $causerId);
+        }
+
+        if ($subjectType = request('subject_type')) {
+            $query->where('subject_type', $subjectType);
+        }
+
+        if ($logName = request('log_name')) {
+            $query->where('log_name', $logName);
+        }
+
+        if ($from = request('from')) {
+            $query->where('created_at', '>=', $from);
+        }
+
+        if ($to = request('to')) {
+            $query->where('created_at', '<=', $to);
+        }
+
+        $activities = $query->paginate(25);
+
+        return response()->json([
+            'data' => $activities->map(fn (Activity $activity) => [
+                'id' => $activity->id,
+                'log_name' => $activity->log_name,
+                'description' => $activity->description,
+                'event' => $activity->event,
+                'causer' => $activity->causer ? [
+                    'id' => $activity->causer->id,
+                    'name' => $activity->causer->full_name ?? $activity->causer->name ?? null,
+                    'email' => $activity->causer->email ?? null,
+                ] : null,
+                'subject_type' => $activity->subject_type,
+                'subject_id' => $activity->subject_id,
+                'properties' => $activity->properties,
+                'created_at' => $activity->created_at->toIso8601String(),
+            ]),
+            'meta' => [
+                'current_page' => $activities->currentPage(),
+                'last_page' => $activities->lastPage(),
+                'per_page' => $activities->perPage(),
+                'total' => $activities->total(),
+            ],
+        ]);
+    }
+}