Add Docker healthchecks so the API waits for Postgres

Why

Once migrations run on startup (see the SQLAlchemy/Alembic issue), the api container will crash-loop if Postgres isn't accepting connections yet. A healthcheck on db plus depends_on with condition: service_healthy makes the api wait.

Files to touch

• docker-compose.yml

Definition of done

• db service has a healthcheck using pg_isready
• api has depends_on: db: condition: service_healthy
• docker compose down -v && docker compose up from a cold start shows no DB connection errors in the api logs
• docker compose ps shows db as healthy

Hints

The postgres image ships with pg_isready. Set a short interval and a few retries. Docs: https://docs.docker.com/reference/compose-file/services/#healthcheck

Start in docker-compose.yml and read the db and api service definitions, then check the Compose healthcheck and depends_on documentation. Run docker compose down -v && docker compose up from a cold start, confirm the api logs show no DB connection errors, and verify with docker compose ps that db is healthy.