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/composables/useEventStorage.ts

@@ -3,7 +3,6 @@ export interface StoredEvent {
   organizerToken?: string
   title: string
   dateTime: string
-  expiryDate: string
   rsvpToken?: string
   rsvpName?: string
 }
@@ -66,7 +65,7 @@ export function useEventStorage() {
       existing.rsvpToken = rsvpToken
       existing.rsvpName = rsvpName
     } else {
-      events.push({ eventToken, title, dateTime, expiryDate: '', rsvpToken, rsvpName })
+      events.push({ eventToken, title, dateTime, rsvpToken, rsvpName })
     }
     writeEvents(events)
   }