Add example fields to CreateEventResponse schema

Required for @msw/source mock generation in E2E tests.
Also good OpenAPI documentation practice.

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

@@ -80,18 +80,23 @@ components:
           type: string
           format: uuid
           description: Public token for the event URL
+          example: "a1b2c3d4-e5f6-7890-abcd-ef1234567890"
         organizerToken:
           type: string
           format: uuid
           description: Secret token for organizer access
+          example: "f9e8d7c6-b5a4-3210-fedc-ba9876543210"
         title:
           type: string
+          example: "Summer BBQ"
         dateTime:
           type: string
           format: date-time
+          example: "2026-03-15T20:00:00+01:00"
         expiryDate:
           type: string
           format: date
+          example: "2026-06-15"
 
     ProblemDetail:
       type: object