Move compose credentials and ports into a .env file

Why

Postgres credentials and ports are hardcoded in docker-compose.yml. A .env file lets everyone configure their local setup without editing tracked files, and mirrors how real deployments handle config.

Files to touch

• docker-compose.yml
• .env.example (new)
• .gitignore

Definition of done

• Compose reads POSTGRES_USER, POSTGRES_PASSWORD, POSTGRES_DB, and any exposed ports from .env
• .env.example is committed with a comment explaining every variable
• .env is gitignored
• cp .env.example .env && docker compose up works on a fresh clone

Hints

Compose automatically loads a .env file from the project root. Use ${VAR} syntax in the YAML. Docs: https://docs.docker.com/compose/environment-variables/

Start with docker-compose.yml and inspect the existing hardcoded Postgres credentials and exposed ports. Read Docker's Compose environment-variable documentation, then update docker-compose.yml, add the documented variables to .env.example, and check .gitignore. Verify the definition of done with cp .env.example .env && docker compose up from a fresh-clone setup.