fix: Notification page breakages (#5236)

- Remove the cascading foreign key indexes
- Add migration to clean up existing objects

fixes: #4285

app/models/conversation.rb

@@ -37,12 +37,6 @@
 #  index_conversations_on_status_and_account_id       (status,account_id)
 #  index_conversations_on_team_id                     (team_id)
 #
-# Foreign Keys
-#
-#  fk_rails_...  (campaign_id => campaigns.id) ON DELETE => cascade
-#  fk_rails_...  (contact_inbox_id => contact_inboxes.id) ON DELETE => cascade
-#  fk_rails_...  (team_id => teams.id) ON DELETE => cascade
-#
 
 class Conversation < ApplicationRecord
   include Labelable
@@ -88,7 +82,7 @@ class Conversation < ApplicationRecord
   has_many :mentions, dependent: :destroy_async
   has_many :messages, dependent: :destroy_async, autosave: true
   has_one :csat_survey_response, dependent: :destroy_async
-  has_many :notifications, as: :primary_actor, dependent: :destroy
+  has_many :notifications, as: :primary_actor, dependent: :destroy_async
 
   before_save :ensure_snooze_until_reset
   before_create :mark_conversation_pending_if_bot