[TASK] Refactor codebase to use one database instead of two

[ustkost]

Description

Change Docker Compose file and backend services to work with just one mutual database instead of two (how it is now) to ease the development process as discussed. Also adapt environment variables

To do

• Modify docker-compose.yml: instead of quizhub-db and quizcore-db services create just one db service
• Some backend services require minor changes: auth and core need to get rid of GORM automigrations
• Change .env example files to reflect the refactored environment variables

Acceptance Criteria

• Only one database service is inside the docker-compose
• Backend services do not conflict while sharing the same database