perf: add composite index for account-scoped conversation created_at sort (#15360)

## Description

Adds a composite index `(account_id, status, created_at)` on
`conversations`.

Sorting an account's conversation list by `created_at` ("newest first")
across all inboxes has no supporting index today, so Postgres falls back
to a backward scan of the global `index_conversations_on_created_at`
filtered by `account_id`. On large databases the planner estimates a
very deep scan and the query can exceed the statement timeout. The
composite index lets the ordered limit be served directly from the
`account_id`/`status` prefix. The default `last_activity_at` sort and
inbox-filtered views are unaffected (already indexed).

The index is created with `algorithm: :concurrently` (no table lock). On
very large installations, create it out-of-band concurrently before
deploying so the migration doesn't run a long build.

Fixes https://linear.app/chatwoot/issue/CW-7888

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

db/migrate/20260807101420_add_account_status_created_at_index_to_conversations.rb

@@ -0,0 +1,12 @@
+class AddAccountStatusCreatedAtIndexToConversations < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def change
+    # Serves account-scoped conversation lists sorted by created_at (e.g. "newest first"),
+    # which otherwise fall back to a backward scan of the global created_at index.
+    add_index :conversations, [:account_id, :status, :created_at],
+              name: 'index_conversations_on_account_id_status_created_at',
+              algorithm: :concurrently,
+              if_not_exists: true
+  end
+end