security: A01-13 — nest all event routes under organisation prefix

Move all authenticated organiser-facing event sub-resource routes from
/events/{event}/... to /organisations/{organisation}/events/{event}/...
to enforce multi-tenancy at the routing layer.

Changes:
- Routes: restructured api.php to nest all event sub-resources under
  the existing organisation prefix group
- Controllers: added Organisation parameter and VerifiesOrganisationEvent
  trait to all 12 affected controllers (sections, time-slots, shifts,
  persons, crowd-lists, locations, shift-assignments, registration-fields,
  availabilities, field-values, section-preferences, stats)
- Tests: updated all 20 feature test files with new route paths
- Frontend: updated 8 API composables and 20 Vue components/pages
- API.md: updated documentation to reflect new route structure

Portal routes, public routes (volunteer-register), and invitation routes
remain unchanged as they operate without organisation context.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/Http/Controllers/Api/V1/EventController.php

@@ -146,9 +146,12 @@ final class EventController extends Controller
         return response()->json(['url' => $event->fresh()->{$field}]);
     }
 
-    public function stats(Event $event): JsonResponse
+    public function stats(Organisation $organisation, Event $event): JsonResponse
     {
-        Gate::authorize('view', $event);
+        if ($event->organisation_id !== $organisation->id) {
+            abort(404);
+        }
+        Gate::authorize('view', [$event, $organisation]);
 
         $personCounts = $event->persons()
             ->selectRaw("