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/tests/Feature/RegistrationFieldTemplate/RegistrationFieldTemplateTest.php

@@ -171,7 +171,7 @@ class RegistrationFieldTemplateTest extends TestCase
 
         Sanctum::actingAs($this->orgAdmin);
 
-        $response = $this->postJson("/api/v1/events/{$event->id}/registration-fields/from-template", [
+        $response = $this->postJson("/api/v1/organisations/{$this->organisation->id}/events/{$event->id}/registration-fields/from-template", [
             'template_id' => $template->id,
         ]);