perf: Add database index on conversations identifier (#12715)

**Problem**
Slack webhook processing was failing with 500 errors due to database
timeouts. The query `Conversation.where(identifier:
params[:event][:thread_ts]).first` was performing full table scans and
hitting PostgreSQL statement timeout.

**Solution**
Added database index on conversations.identifier and account_id.

db/migrate/20251022152158_add_index_to_conversations_identifier.rb

@@ -0,0 +1,6 @@
+class AddIndexToConversationsIdentifier < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+  def change
+    add_index :conversations, [:identifier, :account_id], name: 'index_conversations_on_identifier_and_account_id', algorithm: :concurrently
+  end
+end