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/home-events.spec.ts

@@ -7,7 +7,6 @@ const futureEvent1: StoredEvent = {
   eventToken: 'future-aaa',
   title: 'Summer BBQ',
   dateTime: '2027-06-15T18:00:00Z',
-  expiryDate: '2027-06-16T00:00:00Z',
   organizerToken: 'org-token-1',
 }
 
@@ -15,7 +14,6 @@ const futureEvent2: StoredEvent = {
   eventToken: 'future-bbb',
   title: 'Team Meeting',
   dateTime: '2027-01-10T09:00:00Z',
-  expiryDate: '2027-01-11T00:00:00Z',
   rsvpToken: 'rsvp-token-1',
   rsvpName: 'Alice',
 }
@@ -24,7 +22,6 @@ const pastEvent: StoredEvent = {
   eventToken: 'past-ccc',
   title: 'New Year Party',
   dateTime: '2025-01-01T00:00:00Z',
-  expiryDate: '2025-01-02T00:00:00Z',
 }
 
 function seedEvents(events: StoredEvent[]): string {
@@ -85,7 +82,6 @@ test.describe('US4: Past Events Appear Faded', () => {
           location: '',
           timezone: 'UTC',
           attendeeCount: 0,
-          expired: true,
         })
       }),
     )
@@ -199,13 +195,11 @@ test.describe('Temporal Grouping: Section Headers', () => {
       eventToken: 'today-1',
       title: 'Today Standup',
       dateTime: new Date(now.getFullYear(), now.getMonth(), now.getDate(), 18, 0, 0).toISOString(),
-      expiryDate: '',
     }
     const laterEvent: StoredEvent = {
       eventToken: 'later-1',
       title: 'Future Conference',
       dateTime: new Date(now.getFullYear() + 1, 0, 15, 10, 0, 0).toISOString(),
-      expiryDate: '',
     }
     await page.addInitScript(seedEvents([todayEvent, laterEvent, pastEvent]))
     await page.goto('/')
@@ -245,7 +239,6 @@ test.describe('Temporal Grouping: Section Headers', () => {
       eventToken: 'today-emph',
       title: 'Emphasis Test',
       dateTime: new Date(now.getFullYear(), now.getMonth(), now.getDate(), 20, 0, 0).toISOString(),
-      expiryDate: '',
     }
     await page.addInitScript(seedEvents([todayEvent]))
     await page.goto('/')
@@ -262,7 +255,6 @@ test.describe('Temporal Grouping: Date Subheaders', () => {
       eventToken: 'today-sub',
       title: 'No Subheader Test',
       dateTime: new Date(now.getFullYear(), now.getMonth(), now.getDate(), 19, 0, 0).toISOString(),
-      expiryDate: '',
     }
     await page.addInitScript(seedEvents([todayEvent]))
     await page.goto('/')
@@ -355,7 +347,6 @@ test.describe('US1: View My Events', () => {
           location: '',
           timezone: 'UTC',
           attendeeCount: 0,
-          expired: false,
         })
       }),
     )