Implement cancel-event feature (016)

Add PATCH /events/{eventToken} endpoint for organizers to cancel events,
cancellation banner for visitors, and RSVP rejection on cancelled events.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/e2e/cancelled-event-visitor.spec.ts

@@ -0,0 +1,74 @@
+import { http, HttpResponse } from 'msw'
+import { test, expect } from './msw-setup'
+
+const cancelledEventWithReason = {
+  eventToken: 'a1b2c3d4-e5f6-7890-abcd-ef1234567890',
+  title: 'Summer BBQ',
+  description: 'Bring your own drinks!',
+  dateTime: '2026-03-15T20:00:00+01:00',
+  timezone: 'Europe/Berlin',
+  location: 'Central Park, NYC',
+  attendeeCount: 12,
+  cancelled: true,
+  cancellationReason: 'Venue no longer available',
+}
+
+const cancelledEventWithoutReason = {
+  ...cancelledEventWithReason,
+  cancellationReason: null,
+}
+
+test.describe('US2: Visitor sees cancelled event with reason', () => {
+  test('visitor sees red banner with cancellation reason on cancelled event', async ({
+    page,
+    network,
+  }) => {
+    network.use(
+      http.get('*/api/events/:token', () => HttpResponse.json(cancelledEventWithReason)),
+    )
+
+    await page.goto(`/events/${cancelledEventWithReason.eventToken}`)
+
+    await expect(page.getByText(/This event has been cancelled/i)).toBeVisible()
+    await expect(page.getByText('Venue no longer available')).toBeVisible()
+  })
+})
+
+test.describe('US2: Visitor sees cancelled event without reason', () => {
+  test('visitor sees red banner without reason when no reason was provided', async ({
+    page,
+    network,
+  }) => {
+    network.use(
+      http.get('*/api/events/:token', () => HttpResponse.json(cancelledEventWithoutReason)),
+    )
+
+    await page.goto(`/events/${cancelledEventWithoutReason.eventToken}`)
+
+    await expect(page.getByText(/This event has been cancelled/i)).toBeVisible()
+    // No reason text shown
+    await expect(page.getByText('Venue no longer available')).not.toBeVisible()
+  })
+})
+
+test.describe('US2: RSVP buttons hidden on cancelled event', () => {
+  test('RSVP buttons hidden on cancelled event, other details remain visible', async ({
+    page,
+    network,
+  }) => {
+    network.use(
+      http.get('*/api/events/:token', () => HttpResponse.json(cancelledEventWithReason)),
+    )
+
+    await page.goto(`/events/${cancelledEventWithReason.eventToken}`)
+
+    // Event details are still visible
+    await expect(page.getByRole('heading', { name: 'Summer BBQ' })).toBeVisible()
+    await expect(page.getByText('Bring your own drinks!')).toBeVisible()
+    await expect(page.getByText('Central Park, NYC')).toBeVisible()
+    await expect(page.getByText('12 going')).toBeVisible()
+
+    // RSVP bar is NOT visible
+    await expect(page.getByRole('button', { name: "I'm attending" })).not.toBeVisible()
+  })
+})