refactor(backend): validate string lengths

Requirements
1. Table `subreddits`

| Column | Limit |
| ------------- | ------------- |
| name  |  100 char |
| description  | 300 char  |

2. Table `users`

| Column | Limit |
| ------------- | ------------- |
| email  |  320 char |
| username  | 1000 char  |