docker-compose invalid type, it should be a string
The Docker Compose file reference states, that environment variables are defined as VARIABLE=value array elements. For your case, the docker-compose.yml file would need to be changed to this:

version: '3'
services:
  server:
    ports:
     - 13045:3000
    environment:
     - NODE_CONFIG='{"DATABASE_URL":"http://db:5984"}'