Feature List

• Can users delete cards, boards or lists? Can they edit them? If so add these to your feature list

Backend Routes

• I see you have delete routes in here, you probably also need routes to update a board,list or card.

Frontend Routes

These look good and simple.

Schema

• I think your markdown tables went wonky. Check out the github markdown page for details on how to get them to render properly https://guides.github.com/features/mastering-markdown/
• attribute is spelled incorrectly
• Why do lists have a complete attribute? What does it mean to mark a list as "complete"?
• What is the 'data' field of a Card? Just a big block of text for the card? Maybe this should be the "description"? it should be whatever you call the field on the frontend probably.

Apart from those things this looks simple and clean. I like it.

You might consider adding some wireframes.