Add PostgreSQL to docker-compose.yml and Dockerfile

Add PostgreSQL to docker-compose.yml and configure microservices (user-service, transaction-service, etc.) to use the database.

Tasks:

• Add a PostgreSQL service in docker-compose.yml.
• Create a Docker volume for persistent database storage.
• Expose PostgreSQL on port 5432.
• Update all microservices to use PostgreSQL.
• Set DATABASE_URL as an environment variable in each service.

Acceptance Criteria

• docker-compose.yml includes a PostgreSQL service.
• Database connection works for user-service and transaction-service.
• Database persists data across container restarts.