Add cancel RSVP feature (backend DELETE endpoint + frontend UI)

Allows guests to cancel their RSVP via a DELETE endpoint using their
guestToken. Frontend shows cancel button in RsvpBar and clears local
storage on success. Includes unit tests, integration tests, and E2E spec.

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

frontend/src/views/__tests__/EventCreateView.spec.ts

@@ -163,6 +163,7 @@ describe('EventCreateView', () => {
       getOrganizerToken: vi.fn(),
       saveRsvp: vi.fn(),
       getRsvp: vi.fn(),
+      removeRsvp: vi.fn(),
       removeEvent: vi.fn(),
     })