Make expiryDate an internal concern, auto-set to event date + 7 days

The expiry date is no longer user-facing: it is removed from the API
(request and response) and the frontend. The backend now automatically
calculates it as the event date plus 7 days. The expired banner and
RSVP-bar filtering by expired status are also removed from the UI,
since expiry is purely an internal data-retention mechanism.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/__tests__/EventList.spec.ts

@@ -15,11 +15,11 @@ const router = createRouter({
 const NOW = new Date(2026, 2, 11, 12, 0, 0)
 
 const mockEvents = [
-  { eventToken: 'past-1', title: 'Past Event', dateTime: '2026-03-01T10:00:00', expiryDate: '' },
-  { eventToken: 'later-1', title: 'Later Event', dateTime: '2027-06-15T10:00:00', expiryDate: '' },
-  { eventToken: 'today-1', title: 'Today Event', dateTime: '2026-03-11T18:00:00', expiryDate: '' },
-  { eventToken: 'week-1', title: 'This Week Event', dateTime: '2026-03-13T10:00:00', expiryDate: '' },
-  { eventToken: 'nextweek-1', title: 'Next Week Event', dateTime: '2026-03-16T10:00:00', expiryDate: '' },
+  { eventToken: 'past-1', title: 'Past Event', dateTime: '2026-03-01T10:00:00' },
+  { eventToken: 'later-1', title: 'Later Event', dateTime: '2027-06-15T10:00:00' },
+  { eventToken: 'today-1', title: 'Today Event', dateTime: '2026-03-11T18:00:00' },
+  { eventToken: 'week-1', title: 'This Week Event', dateTime: '2026-03-13T10:00:00' },
+  { eventToken: 'nextweek-1', title: 'Next Week Event', dateTime: '2026-03-16T10:00:00' },
 ]
 
 vi.mock('../../composables/useEventStorage', () => ({

frontend/src/components/__tests__/useEventGrouping.spec.ts

@@ -6,7 +6,6 @@ function makeEvent(overrides: Partial<StoredEvent> & { dateTime: string }): Stor
   return {
     eventToken: `evt-${Math.random().toString(36).slice(2, 8)}`,
     title: 'Test Event',
-    expiryDate: '',
     ...overrides,
   }
 }