chore: update create_dummy_meeting to specify session purposes

[jennifer-richards]

Also broken on the main branch, but fixing on feat/tzaware because no one is likely to run the command on main until after the tzaware switch.

[rjsparks]

Not a priority, but this looks like a place where using factories would be way better than bare ORM create calls. Could ditch the manual effort of looking up/encoding group pks for example...

[jennifer-richards]

Created #4657 to capture the factory refactoring