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/views/__tests__/EventDetailView.spec.ts

@@ -54,7 +54,6 @@ const fullEvent = {
   timezone: 'Europe/Berlin',
   location: 'Central Park, NYC',
   attendeeCount: 12,
-  expired: false,
 }
 
 function mockLoadedEvent(eventOverrides = {}) {
@@ -124,29 +123,6 @@ describe('EventDetailView', () => {
     wrapper.unmount()
   })
 
-  // Expired state
-  it('renders "event has ended" banner when expired', async () => {
-    mockLoadedEvent({ expired: true })
-
-    const wrapper = await mountWithToken()
-    await flushPromises()
-
-    expect(wrapper.text()).toContain('This event has ended.')
-    expect(wrapper.find('.detail__banner--expired').exists()).toBe(true)
-    wrapper.unmount()
-  })
-
-  // No expired banner when not expired
-  it('does not render expired banner when event is active', async () => {
-    mockLoadedEvent()
-
-    const wrapper = await mountWithToken()
-    await flushPromises()
-
-    expect(wrapper.find('.detail__banner--expired').exists()).toBe(false)
-    wrapper.unmount()
-  })
-
   // Not found state
   it('renders "event not found" when API returns 404', async () => {
     vi.mocked(api.GET).mockResolvedValue({
@@ -229,17 +205,6 @@ describe('EventDetailView', () => {
     wrapper.unmount()
   })
 
-  it('does not show RSVP bar on expired event', async () => {
-    mockLoadedEvent({ expired: true })
-
-    const wrapper = await mountWithToken()
-    await flushPromises()
-
-    expect(wrapper.find('.rsvp-bar__cta').exists()).toBe(false)
-    expect(wrapper.find('.rsvp-bar').exists()).toBe(false)
-    wrapper.unmount()
-  })
-
   it('shows RSVP status bar when localStorage has RSVP', async () => {
     mockGetRsvp.mockReturnValue({ rsvpToken: 'rsvp-1', rsvpName: 'Max' })
     mockLoadedEvent()