chore: Improved indexes for Conversations & Contacts [CW-3300] (#9475)

Based on our recent performant optimisation exercises, We have identified a better indexing strategy for conversations and contacts. The previous index on last_activity_at for conversations significantly slowed down conversation filters. Similarly, the new index on Contacts will allow the page rendering to improve for accounts with many contacts.

fixes: https://linear.app/chatwoot/issue/CW-3300/db-improvements

db/migrate/20240515201632_index_improvements_conversations_contacts.rb

@@ -0,0 +1,11 @@
+class IndexImprovementsConversationsContacts < ActiveRecord::Migration[7.0]
+  disable_ddl_transaction!
+
+  def change
+    remove_index :conversations, :last_activity_at
+    add_index :contacts, [:account_id, :last_activity_at],
+              order: { last_activity_at: 'DESC NULLS LAST' },
+              algorithm: :concurrently,
+              name: 'index_contacts_on_account_id_and_last_activity_at'
+  end
+end