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__/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,
   }
 }