Add DB integration

[Jacopo Farina]

Once the scaffolding in #2 is present, the backend should be able to use a DB containing the quiz data.

Create an endpoint which retrieves a random card for a language pair. The DB connection string should be passed an environment variable, and the connection pool started with the app, then a new connection is retrieved when needed in the endpoint code using the pool.

acceptance criteria:

• given an initialized DB the endpoint works
• an integration test can check this behavior
• the CI pipeline can run an integration test using sample data (see https://github.com/MagicStack/asyncpg for an example implementation)