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>

specs/014-cancel-rsvp/quickstart.md

@@ -0,0 +1,57 @@
+# Quickstart: Cancel RSVP
+
+**Feature**: 014-cancel-rsvp | **Date**: 2026-03-09
+
+## Implementation Order
+
+1. **OpenAPI spec** — Add DELETE endpoint to `api.yaml`
+2. **Regenerate types** — Backend (Maven generate) + Frontend (`npm run generate:api`)
+3. **Backend TDD** — Write tests → implement repository → service → controller
+4. **Frontend composable** — Add `removeRsvp()` to `useEventStorage.ts`
+5. **Frontend US-1** — RsvpBar tap-to-reveal + EventDetailView cancel logic
+6. **Frontend US-2** — EventList conditional dialog + server-side cancel before removal
+7. **Frontend US-3** — Edge case handling (stale tokens treated as success)
+8. **E2E tests** — Playwright tests for all three user stories
+
+## Key Commands
+
+```bash
+# Backend
+cd backend && ./mvnw test                    # Run backend tests
+cd backend && ./mvnw verify                  # Full verify (checkstyle + tests)
+cd backend && ./mvnw spring-boot:run         # Run backend locally
+
+# Frontend
+cd frontend && npm run generate:api          # Regenerate types from OpenAPI
+cd frontend && npm run test:unit             # Run unit tests
+cd frontend && npx playwright test           # Run E2E tests
+cd frontend && npm run dev                   # Dev server
+
+# Both
+cd backend && ./mvnw test && cd ../frontend && npm run test:unit  # Quick check
+```
+
+## Key Files to Modify
+
+| File | Change |
+|------|--------|
+| `backend/src/main/resources/openapi/api.yaml` | Add DELETE `/events/{token}/rsvps/{rsvpToken}` |
+| `backend/.../port/in/CancelRsvpUseCase.java` | New use case interface |
+| `backend/.../port/out/RsvpRepository.java` | Add `deleteByEventIdAndRsvpToken()` |
+| `backend/.../RsvpJpaRepository.java` | Add derived delete query |
+| `backend/.../RsvpPersistenceAdapter.java` | Implement delete |
+| `backend/.../RsvpService.java` | Implement `CancelRsvpUseCase` |
+| `backend/.../EventController.java` | Add `cancelRsvp()` handler |
+| `frontend/src/composables/useEventStorage.ts` | Add `removeRsvp()` |
+| `frontend/src/components/RsvpBar.vue` | Tap-to-reveal cancel button |
+| `frontend/src/views/EventDetailView.vue` | Cancel logic + confirm dialog |
+| `frontend/src/components/EventList.vue` | Conditional dialog message + server cancel |
+| `frontend/e2e/cancel-rsvp.spec.ts` | E2E tests for all scenarios |
+
+## Gotchas
+
+- The JPA `deleteBy...` method requires `@Transactional` on the calling service method.
+- The `@Transactional` import must be `jakarta.transaction.Transactional` (not Spring's).
+- After updating OpenAPI spec, run `npm run generate:api` in frontend AND Maven generate-sources in backend.
+- The RsvpBar component currently has no click handler on the status state — this needs to be added carefully with proper accessibility (role, aria-expanded, keyboard support).
+- The EventList's `confirmDelete` currently calls `removeEvent()` synchronously — it needs to become async for the server call.