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/__tests__/useEventStorage.spec.ts

@@ -43,7 +43,6 @@ describe('useEventStorage', () => {
       organizerToken: 'org-456',
       title: 'Birthday',
       dateTime: '2026-06-15T20:00:00+02:00',
-      expiryDate: '2026-07-15',
     })
 
     const events = getStoredEvents()
@@ -61,7 +60,6 @@ describe('useEventStorage', () => {
       organizerToken: 'org-456',
       title: 'Test',
       dateTime: '2026-06-15T20:00:00+02:00',
-      expiryDate: '2026-07-15',
     })
 
     expect(getOrganizerToken('abc-123')).toBe('org-456')
@@ -79,14 +77,12 @@ describe('useEventStorage', () => {
       eventToken: 'event-1',
       title: 'First',
       dateTime: '2026-06-15T20:00:00+02:00',
-      expiryDate: '2026-07-15',
     })
 
     saveCreatedEvent({
       eventToken: 'event-2',
       title: 'Second',
       dateTime: '2026-07-15T20:00:00+02:00',
-      expiryDate: '2026-08-15',
     })
 
     const events = getStoredEvents()
@@ -102,14 +98,12 @@ describe('useEventStorage', () => {
       eventToken: 'abc-123',
       title: 'Old Title',
       dateTime: '2026-06-15T20:00:00+02:00',
-      expiryDate: '2026-07-15',
     })
 
     saveCreatedEvent({
       eventToken: 'abc-123',
       title: 'New Title',
       dateTime: '2026-06-15T20:00:00+02:00',
-      expiryDate: '2026-07-15',
     })
 
     const events = getStoredEvents()
@@ -124,7 +118,6 @@ describe('useEventStorage', () => {
       eventToken: 'abc-123',
       title: 'Birthday',
       dateTime: '2026-06-15T20:00:00+02:00',
-      expiryDate: '2026-07-15',
     })
 
     saveRsvp('abc-123', 'rsvp-token-1', 'Max', 'Birthday', '2026-06-15T20:00:00+02:00')
@@ -154,7 +147,6 @@ describe('useEventStorage', () => {
       eventToken: 'abc-123',
       title: 'Test',
       dateTime: '2026-06-15T20:00:00+02:00',
-      expiryDate: '2026-07-15',
     })
 
     expect(getRsvp('abc-123')).toBeUndefined()
@@ -172,14 +164,12 @@ describe('useEventStorage', () => {
       eventToken: 'event-1',
       title: 'First',
       dateTime: '2026-06-15T20:00:00+02:00',
-      expiryDate: '2026-07-15',
     })
 
     saveCreatedEvent({
       eventToken: 'event-2',
       title: 'Second',
       dateTime: '2026-07-15T20:00:00+02:00',
-      expiryDate: '2026-08-15',
     })
 
     removeEvent('event-1')
@@ -196,7 +186,6 @@ describe('useEventStorage', () => {
       eventToken: 'event-1',
       title: 'First',
       dateTime: '2026-06-15T20:00:00+02:00',
-      expiryDate: '2026-07-15',
     })
 
     removeEvent('nonexistent')
@@ -220,8 +209,7 @@ describe('isValidStoredEvent', () => {
         eventToken: 'abc-123',
         title: 'Birthday',
         dateTime: '2026-06-15T20:00:00+02:00',
-        expiryDate: '2026-07-15',
-      }),
+        }),
     ).toBe(true)
   })