Review Stage 1 Backend Exercise

Anh @hieunmce review code stage 1 giúp em nha.

[Repo](https://github.com/npkhoa2197/example-go)

- CRUD for book, category, lend book action
- Validations for Create/Update book, category
- Delete book(s) in a category on that category deletion

**TODO**

- Stage 2
- Optional features
