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>

apps/app/src/components/event/ImportFromEventDialog.vue

@@ -17,7 +17,7 @@ const orgIdRef = computed(() => props.orgId)
 const eventIdRef = computed(() => props.eventId)
 
 const { data: events, isLoading: isLoadingEvents } = useEventList(orgIdRef)
-const { mutate: importFields, isPending } = useImportFieldsFromEvent(eventIdRef)
+const { mutate: importFields, isPending } = useImportFieldsFromEvent(orgIdRef, eventIdRef)
 
 const selectedEventId = ref<string | null>(null)
 

apps/app/src/components/event/TemplatePickerDialog.vue

@@ -20,7 +20,7 @@ const orgIdRef = computed(() => props.orgId)
 const eventIdRef = computed(() => props.eventId)
 
 const { data: templates, isLoading } = useRegistrationFieldTemplates(orgIdRef)
-const { mutate: createFromTemplate, isPending } = useCreateFieldFromTemplate(eventIdRef)
+const { mutate: createFromTemplate, isPending } = useCreateFieldFromTemplate(orgIdRef, eventIdRef)
 
 const existingSlugs = computed(() =>
   new Set(props.existingFields.map(f => f.slug)),