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-create.spec.ts

@@ -9,7 +9,6 @@ test.describe('US-1: Create an event', () => {
 
     await expect(page.getByText('Title is required.')).toBeVisible()
     await expect(page.getByText('Date and time are required.')).toBeVisible()
-    await expect(page.getByText('Expiry date is required.')).toBeVisible()
   })
 
   test('creates an event and redirects to event detail page', async ({ page }) => {
@@ -19,7 +18,6 @@ test.describe('US-1: Create an event', () => {
     await page.getByLabel(/description/i).fill('Bring your own drinks')
     await page.getByLabel(/date/i).first().fill('2026-04-15T18:00')
     await page.getByLabel(/location/i).fill('Central Park')
-    await page.getByLabel(/expiry/i).fill('2026-06-15')
 
     await page.getByRole('button', { name: /create event/i }).click()
 
@@ -31,7 +29,6 @@ test.describe('US-1: Create an event', () => {
 
     await page.getByLabel(/title/i).fill('Summer BBQ')
     await page.getByLabel(/date/i).first().fill('2026-04-15T18:00')
-    await page.getByLabel(/expiry/i).fill('2026-06-15')
 
     await page.getByRole('button', { name: /create event/i }).click()
     await expect(page).toHaveURL(/\/events\/.+/)
@@ -59,7 +56,6 @@ test.describe('US-1: Create an event', () => {
     await page.goto('/create')
     await page.getByLabel(/title/i).fill('Test')
     await page.getByLabel(/date/i).first().fill('2026-04-15T18:00')
-    await page.getByLabel(/expiry/i).fill('2026-06-15')
 
     await page.getByRole('button', { name: /create event/i }).click()