feat: account calls dashboard index endpoint (#14780)

## Description

Adds a backend endpoint that powers an account-wide calls dashboard,
letting users list and filter all calls in the account.

## Linear Ticket
- https://linear.app/chatwoot/issue/UPM-28/voice-call-dashboard-view

## Type of change

- [ ] New feature (non-breaking change which adds functionality)

## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

db/migrate/20260622000000_add_account_created_at_index_to_calls.rb

@@ -0,0 +1,7 @@
+class AddAccountCreatedAtIndexToCalls < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def change
+    add_index :calls, [:account_id, :created_at], algorithm: :concurrently
+  end
+end

db/schema.rb

@@ -282,6 +282,7 @@ ActiveRecord::Schema[7.1].define(version: 2026_06_30_000000) do
     t.datetime "updated_at", null: false
     t.index ["account_id", "contact_id"], name: "index_calls_on_account_id_and_contact_id"
     t.index ["account_id", "conversation_id"], name: "index_calls_on_account_id_and_conversation_id"
+    t.index ["account_id", "created_at"], name: "index_calls_on_account_id_and_created_at"
     t.index ["message_id"], name: "index_calls_on_message_id"
     t.index ["provider", "provider_call_id"], name: "index_calls_on_provider_and_provider_call_id", unique: true
   end