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

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: 2026_08_06_000000) do
+ActiveRecord::Schema[7.1].define(version: 2026_08_07_101420) do
   # These extensions should be enabled to support this database
   enable_extension "pg_stat_statements"
   enable_extension "pg_trgm"
@@ -854,6 +854,7 @@ ActiveRecord::Schema[7.1].define(version: 2026_08_06_000000) do
     t.index ["account_id", "display_id"], name: "index_conversations_on_account_id_and_display_id", unique: true
     t.index ["account_id", "id"], name: "index_conversations_on_id_and_account_id"
     t.index ["account_id", "inbox_id", "status", "assignee_id"], name: "conv_acid_inbid_stat_asgnid_idx"
+    t.index ["account_id", "status", "created_at"], name: "index_conversations_on_account_id_status_created_at"
     t.index ["account_id"], name: "index_conversations_on_account_id"
     t.index ["assignee_id", "account_id"], name: "index_conversations_on_assignee_id_and_account_id"
     t.index ["campaign_id"], name: "index_conversations_on_campaign_id"