GET /api/posts returns all posts from the database as an array.
Each post object in the array has the following properties:
- id: Automatically generated ID of the post
- user: Username of the user that created the post
- title: Title of the post
- text: Content of the post
- category: Optional; category of the post
- created: Automatically set to the timestamp when the post is created
- updated: Automatically set to the timestamp when the post is last edited (or created if it was never edited)