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>

backend/src/main/resources/openapi/api.yaml

@@ -184,6 +184,58 @@ paths:
               schema:
                 $ref: "#/components/schemas/ProblemDetail"
 
+    patch:
+      operationId: patchEvent
+      summary: Update an event (currently cancel)
+      description: |
+        Partial update of an event resource. Currently the only supported operation
+        is cancellation (setting cancelled to true). Requires the organizer token.
+        Cancellation is irreversible.
+      tags:
+        - events
+      parameters:
+        - name: eventToken
+          in: path
+          required: true
+          schema:
+            type: string
+            format: uuid
+          description: Public event token
+        - name: organizerToken
+          in: query
+          required: true
+          schema:
+            type: string
+            format: uuid
+          description: Organizer token for authorization
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/PatchEventRequest"
+      responses:
+        "204":
+          description: Event updated successfully
+        "403":
+          description: Invalid organizer token
+          content:
+            application/problem+json:
+              schema:
+                $ref: "#/components/schemas/ProblemDetail"
+        "404":
+          description: Event not found
+          content:
+            application/problem+json:
+              schema:
+                $ref: "#/components/schemas/ProblemDetail"
+        "409":
+          description: Event is already cancelled
+          content:
+            application/problem+json:
+              schema:
+                $ref: "#/components/schemas/ProblemDetail"
+
 components:
   schemas:
     CreateEventRequest:
@@ -252,6 +304,7 @@ components:
         - dateTime
         - timezone
         - attendeeCount
+        - cancelled
       properties:
         eventToken:
           type: string
@@ -284,6 +337,31 @@ components:
           minimum: 0
           description: Number of confirmed attendees (attending=true)
           example: 12
+        cancelled:
+          type: boolean
+          description: Whether the event has been cancelled
+          example: false
+        cancellationReason:
+          type:
+            - string
+            - "null"
+          description: Reason for cancellation, if provided
+          example: null
+
+    PatchEventRequest:
+      type: object
+      required:
+        - cancelled
+      properties:
+        cancelled:
+          type: boolean
+          description: Set to true to cancel the event (irreversible)
+          example: true
+        cancellationReason:
+          type: string
+          maxLength: 2000
+          description: Optional cancellation reason
+          example: "Unfortunately the venue is no longer available."
 
     CreateRsvpRequest:
       type: object