refactor: use short-lived sessions when accessing the database
Previously the database connection ("DB.session") was established during the startup of Aura Engine. This was causing conflicts in the threaded execution environment (#75 (closed)). 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 (closed)