Reset the playground database on every run

[rphlmr]

I think it’s better to clean the embedded database on every run (in-memory vs. file).

It makes it clear that you will only see outputs related to what you have coded.

For SQLite, we have no choice for now due to some limitations (#10).
For Postgres, we could add an option to enable persistence across runs.

Let's keep it simple for now.

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For performance optimization, we still keep the step that prepares the migrations. It enables almost instant database re-creation.