Clear up dependency destruction chain

Buckets destroy their bubbles, which destroy their thread entries, which destroy their threadables, which is how comments end up destroyed, and then rollups end up deleting their events

app/models/thread_entry.rb

@@ -1,7 +1,7 @@
 class ThreadEntry < ApplicationRecord
   belongs_to :bubble
 
-  delegated_type :threadable, types: %w[ Comment Rollup ]
+  delegated_type :threadable, types: %w[ Comment Rollup ], dependent: :destroy
 
   scope :chronologically, -> { order created_at: :asc, id: :desc }
 end