Add delay to event

- [ ] Add delay to event model
- [ ] Add delay to API when creating the delay
- [ ] Use the delay to calculate delay and return to fronend

Start by locating the event model and the API endpoint used when creating a delay, then trace how event data is returned to the frontend. Clarify the intended delay representation and calculation before making changes. Done means the model and API support delay and the calculated delay is returned to the frontend.

索引モデルが issue の本文から書いたものです。