Add REST endpoints to create/list/edit/delete Circles

Since Circles are the focal point of our data model, we need to add REST endpoints to create, read, update, and delete Circles.


```[tasklist]
### Tasks
- [ ] Add endpoint to create a Circle (POST)
- [ ] Add endpoint to read a Circle (GET)
- [ ] Add endpoint to update a Circle (PUT)
- [ ] Add endpoint to delete a Circle (DELETE)
```
