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). 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

src/scheduling/fallback.py

@@ -138,7 +138,8 @@ class FallbackManager:
             if fallback_type != self.state.get("previous_fallback_type"):
                 timeslot = self.state["timeslot"]
                 if timeslot:
-                    DB.session.merge(timeslot)
+                    with DB.Session() as session:
+                        session.merge(timeslot)
                 self.engine.event_dispatcher.on_fallback_active(timeslot, fallback_type)