Thanks, codespell!

An sqlite database is much easier to setup for simple local deployments
and specifically for test automation.

Previously the database connection ("DB.session") was established during
the startup of Aura Engine.
This was causing conflicts in the threaded execution environment (#75).
Now all sessions are acquired in a short-lived context:

  with DB.Session() as session:
      ...

If database query performance is a real concern, then the session
contexts could be moved to a higher level.

Closes: #75