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/VolunteerAvailabilityController.php

@@ -5,8 +5,10 @@ declare(strict_types=1);
 namespace App\Http\Controllers\Api\V1;
 
 use App\Http\Controllers\Controller;
+use App\Http\Controllers\Api\V1\Traits\VerifiesOrganisationEvent;
 use App\Http\Requests\Api\V1\SyncVolunteerAvailabilityRequest;
 use App\Models\Event;
+use App\Models\Organisation;
 use App\Models\Person;
 use App\Models\TimeSlot;
 use App\Models\VolunteerAvailability;
@@ -16,8 +18,11 @@ use Illuminate\Validation\ValidationException;
 
 final class VolunteerAvailabilityController extends Controller
 {
-    public function index(Event $event, Person $person): JsonResponse
+    use VerifiesOrganisationEvent;
+
+    public function index(Organisation $organisation, Event $event, Person $person): JsonResponse
     {
+        $this->verifyEventBelongsToOrganisation($organisation, $event);
         Gate::authorize('view', [$person, $event]);
 
         $availabilities = VolunteerAvailability::where('person_id', $person->id)
@@ -43,8 +48,9 @@ final class VolunteerAvailabilityController extends Controller
     /**
      * @throws ValidationException
      */
-    public function sync(SyncVolunteerAvailabilityRequest $request, Event $event, Person $person): JsonResponse
+    public function sync(SyncVolunteerAvailabilityRequest $request, Organisation $organisation, Event $event, Person $person): JsonResponse
     {
+        $this->verifyEventBelongsToOrganisation($organisation, $event);
         Gate::authorize('update', [$person, $event]);
 
         $availabilities = $request->validated('availabilities');