Set up SQLAlchemy + Alembic with a User model

Why

This is the first real database code and it establishes how models and migrations work for every feature after it.

Depends on: the healthchecks issue (api must wait for Postgres before running migrations).

Files to touch

• backend/app/db.py (new): engine + session
• backend/app/models.py (new): User model
• backend/alembic/ (new): migration environment
• backend/requirements.txt
• api service command in docker-compose.yml

Definition of done

• User model with id, email (unique, indexed), password_hash, created_at
• Database URL is built from the env vars in .env
• alembic upgrade head runs automatically when the api container starts
• After docker compose up, a users table exists in Postgres (verify with docker compose exec db psql -U $POSTGRES_USER -d $POSTGRES_DB -c '\dt')
• CI workflow updated with a Postgres service so tests still pass

Hints

Run alembic init alembic inside the backend container, then point env.py at the app's metadata. Alembic tutorial: https://alembic.sqlalchemy.org/en/latest/tutorial.html

Start with backend/requirements.txt, backend/app/db.py, backend/app/models.py, and the Alembic tutorial; then inspect the api command in docker-compose.yml and the existing CI workflow. Set up the migration environment and User model, run migrations on container startup, and verify the users table with the provided docker compose exec command while ensuring CI tests still pass with Postgres.