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

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

specs/016-cancel-event/data-model.md

@@ -0,0 +1,82 @@
+# Data Model: Cancel Event
+
+**Feature Branch**: `016-cancel-event` | **Date**: 2026-03-12
+
+## Entity Changes
+
+### Event (extended)
+
+Two new fields added to the existing Event entity:
+
+| Field              | Type           | Constraints                  | Description                                      |
+|--------------------|----------------|------------------------------|--------------------------------------------------|
+| cancelled          | boolean        | NOT NULL, DEFAULT false      | Whether the event has been cancelled              |
+| cancellationReason | String (2000)  | Nullable                     | Optional reason provided by organizer             |
+
+### State Transition
+
+```
+ACTIVE ──cancel()──► CANCELLED
+```
+
+- One-way transition only. No path from CANCELLED back to ACTIVE.
+- `cancel()` sets `cancelled = true` and optionally sets `cancellationReason`.
+- Once cancelled, the event remains visible but RSVP creation is blocked.
+
+### Validation Rules
+
+- `cancellationReason` max length: 2000 characters (matches description field).
+- `cancellationReason` is plain text only (no HTML/markdown).
+- `cancelled` can only transition from `false` to `true`, never back.
+- Existing RSVPs are preserved when an event is cancelled (no cascade).
+
+## Database Migration (Liquibase Changeset 004)
+
+```xml
+<changeSet id="004-add-cancellation-columns" author="fete">
+    <addColumn tableName="events">
+        <column name="cancelled" type="BOOLEAN" defaultValueBoolean="false">
+            <constraints nullable="false"/>
+        </column>
+        <column name="cancellation_reason" type="VARCHAR(2000)"/>
+    </addColumn>
+</changeSet>
+```
+
+## Domain Model Impact
+
+### Event.java (domain)
+
+Add fields:
+```java
+private boolean cancelled;
+private String cancellationReason;
+```
+
+Add method:
+```java
+public void cancel(String reason) {
+    if (this.cancelled) {
+        throw new EventAlreadyCancelledException();
+    }
+    this.cancelled = true;
+    this.cancellationReason = reason;
+}
+```
+
+### EventJpaEntity.java (persistence)
+
+Add columns:
+```java
+@Column(name = "cancelled", nullable = false)
+private boolean cancelled;
+
+@Column(name = "cancellation_reason", length = 2000)
+private String cancellationReason;
+```
+
+## RSVP Impact
+
+- `POST /events/{eventToken}/rsvps` must check `event.isCancelled()` before accepting.
+- If cancelled → return `409 Conflict`.
+- Existing RSVPs remain untouched — no delete, no status change.