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.

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2025_06_20_120000) do
+ActiveRecord::Schema[7.1].define(version: 2025_06_27_195529) do
   # These extensions should be enabled to support this database
   enable_extension "pg_stat_statements"
   enable_extension "pg_trgm"
@@ -825,6 +825,7 @@ ActiveRecord::Schema[7.1].define(version: 2025_06_20_120000) do
     t.text "processed_message_content"
     t.jsonb "sentiment", default: {}
     t.index "((additional_attributes -> 'campaign_id'::text))", name: "index_messages_on_additional_attributes_campaign_id", using: :gin
+    t.index ["account_id", "content_type", "created_at"], name: "idx_messages_account_content_created"
     t.index ["account_id", "created_at", "message_type"], name: "index_messages_on_account_created_type"
     t.index ["account_id", "inbox_id"], name: "index_messages_on_account_id_and_inbox_id"
     t.index ["account_id"], name: "index_messages_on_account_id"