Refactor and reorganize Alembic and database configuration.

Moved Alembic files into the `app/alembic` directory and updated related paths. Consolidated database configuration in `config.py`, leveraging environment variables and ensuring centralized management. Updated Docker Compose to include `.env` files, providing a more consistent environment setup.

docker-compose.dev.yml

@@ -28,6 +28,8 @@ services:
       - backend_dev_modules:/app/.venv
     ports:
       - "8000:8000"
+    env_file:
+      - .env
     environment:
       - DATABASE_URL=${DATABASE_URL}
       - SECRET_KEY=${SECRET_KEY}