Might be ok for some events not to have rollups

e.g. if we add events that don't get rolled up

db/migrate/20241022180133_add_rollup_reference_to_events.rb

@@ -1,6 +1,5 @@
 class AddRollupReferenceToEvents < ActiveRecord::Migration[8.0]
   def up
-    # FIXME: add null: false in another migration after data migrations happen
     add_reference :events, :rollup, foreign_key: true
   end
 end