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>

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

@@ -37,6 +37,38 @@ paths:
               schema:
                 $ref: "#/components/schemas/ValidationProblemDetail"
 
+  /events/{token}/rsvps/{rsvpToken}:
+    delete:
+      operationId: cancelRsvp
+      summary: Cancel RSVP
+      description: |
+        Permanently deletes an RSVP identified by the RSVP token.
+        Idempotent: returns 204 whether the RSVP existed or not.
+      tags:
+        - events
+      parameters:
+        - name: token
+          in: path
+          required: true
+          schema:
+            type: string
+            format: uuid
+          description: Event token (UUID)
+        - name: rsvpToken
+          in: path
+          required: true
+          schema:
+            type: string
+            format: uuid
+          description: RSVP token (UUID) identifying the attendance to cancel
+      responses:
+        "204":
+          description: >
+            RSVP successfully cancelled (or was already cancelled).
+            No response body.
+        "500":
+          description: Internal server error
+
   /events/{token}/rsvps:
     post:
       operationId: createRsvp