Switch from SQLite to Postgres locally

[agorajek]

To make sure that Postgres compatibility and performance is tested extensively, since we want to call it as our production DJ database.

[shangyian]

One last thing needed here is to switch from SQLite in the unit tests over to Postgres. This will make it so that our unit tests are a bit closer to testing what will actually happen upon deployment with a Postgres metadata database. We can use testcontainers to bring up the Postgres image.