feat(organisation): add dashboard-stats endpoint

GET /organisations/{organisation}/dashboard-stats returns members,
events (with status breakdown + active count), persons, the first five
members sorted by join date, and the five most recent activity log
entries. Business logic lives in OrganisationDashboardService; access
follows OrganisationPolicy@view.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/routes/api.php

@@ -168,6 +168,7 @@ Route::middleware(['auth:sanctum', 'impersonation'])->group(function () {
     // Organisations
     Route::apiResource('organisations', OrganisationController::class)
         ->only(['index', 'show', 'store', 'update']);
+    Route::get('organisations/{organisation}/dashboard-stats', [OrganisationController::class, 'dashboardStats']);
 
     // Events (nested under organisations)
     Route::apiResource('organisations.events', EventController::class)