Add spec, plan, and tasks for 016-cancel-event feature

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

specs/016-cancel-event/quickstart.md

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+# Quickstart: Cancel Event
+
+**Feature Branch**: `016-cancel-event`
+
+## What This Feature Does
+
+Adds the ability for an organizer to permanently cancel an event. Cancelled events display a red banner to visitors and block new RSVPs.
+
+## Implementation Scope
+
+### Backend
+1. **Liquibase migration** (003): Add `cancelled` (boolean) and `cancellation_reason` (varchar 2000) columns to `events` table.
+2. **Domain model**: Extend `Event.java` with `cancelled` and `cancellationReason` fields + `cancel()` method.
+3. **JPA entity**: Extend `EventJpaEntity.java` with matching columns and mapper updates.
+4. **OpenAPI spec**: Add `PATCH /events/{eventToken}` endpoint + extend `GetEventResponse` with cancellation fields.
+5. **Use case**: New `CancelEventUseCase` interface + implementation in `EventService`.
+6. **Controller**: Implement `cancelEvent` in `EventController`.
+7. **RSVP guard**: Add cancelled check to RSVP creation (return 409).
+
+### Frontend
+1. **Cancel bottom sheet**: Add cancel button (organizer-only) + bottom sheet with textarea and confirm button in `EventDetailView.vue`.
+2. **Cancellation banner**: Red banner at top of event detail when `cancelled === true`.
+3. **RSVP hiding**: Hide `RsvpBar` when event is cancelled.
+4. **API client**: Use generated types from updated OpenAPI spec.
+
+### Testing
+1. **Backend unit tests**: Cancel use case, RSVP rejection on cancelled events.
+2. **Backend integration tests**: Full cancel flow via API.
+3. **Frontend unit tests**: Cancel bottom sheet, banner display, RSVP hiding.
+4. **E2E tests**: Organizer cancels event, attendee sees cancelled event.
+
+## Key Files to Modify
+
+| File | Change |
+|------|--------|
+| `backend/src/main/resources/openapi/api.yaml` | New endpoint + schema extensions |
+| `backend/src/main/resources/db/changelog/` | New changeset 003 |
+| `backend/src/main/java/de/fete/domain/model/Event.java` | Add cancelled fields + cancel() |
+| `backend/src/main/java/de/fete/adapter/out/persistence/EventJpaEntity.java` | Add columns |
+| `backend/src/main/java/de/fete/application/service/EventService.java` | Implement cancel |
+| `backend/src/main/java/de/fete/adapter/in/web/EventController.java` | Implement endpoint |
+| `frontend/src/views/EventDetailView.vue` | Cancel button, bottom sheet, banner |
+
+## Prerequisites
+
+- Existing RSVP bottom sheet pattern (already implemented)
+- Organizer token stored in localStorage (already implemented)
+- `BottomSheet.vue` component (already exists)