fix: Add composite index on messages for csat_metrics API performance (#11831)

This PR adds a composite index (:account_id, :content_type, :created_at)
on the table messages.

This index is added as a temporary fix for performance issues in the
CSAT responses controller where we query messages with account_id,
content_type and created_at. The current implementation
(account.message.input_csat.count) times out with millions of messages.

TODO: Create a dedicated csat_survey table and add entries when surveys
are sent, then query this table instead of the entire messages table for
better performance.

app/models/message.rb

@@ -24,6 +24,7 @@
 #
 # Indexes
 #
+#  idx_messages_account_content_created                 (account_id,content_type,created_at)
 #  index_messages_on_account_created_type               (account_id,created_at,message_type)
 #  index_messages_on_account_id                         (account_id)
 #  index_messages_on_account_id_and_inbox_id            (account_id,inbox_id)