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/e2e/event-rsvp.spec.ts

@@ -9,7 +9,6 @@ const fullEvent = {
   timezone: 'Europe/Berlin',
   location: 'Central Park, NYC',
   attendeeCount: 12,
-  expired: false,
 }
 
 test.describe('US1: RSVP submission flow', () => {
@@ -170,16 +169,4 @@ test.describe('US1: RSVP submission flow', () => {
     await expect(page.getByText("You're attending!")).not.toBeVisible()
   })
 
-  test('does not show RSVP bar on expired event', async ({ page, network }) => {
-    network.use(
-      http.get('*/api/events/:token', () => {
-        return HttpResponse.json({ ...fullEvent, expired: true })
-      }),
-    )
-
-    await page.goto(`/events/${fullEvent.eventToken}`)
-
-    await expect(page.getByText('This event has ended.')).toBeVisible()
-    await expect(page.getByRole('button', { name: "I'm attending" })).not.toBeVisible()
-  })
 })