chore(voice): drop non-UI changes from feat/whatsapp-call-ui

Reset BE/migration/spec files to base so this PR diffs as a pure UI change.
The deleted migration and the BE-side comment/edits belong on the BE PR
chain (feat/whatsapp-call-incoming-pipeline) rather than the UI PR.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/controllers/api/v1/accounts/contacts/conversations_controller.rb

@@ -4,10 +4,6 @@ class Api::V1::Accounts::Contacts::ConversationsController < Api::V1::Accounts::
     conversations = Current.account.conversations.includes(
       :assignee, :contact, :inbox, :taggings
     ).where(contact_id: @contact.id)
-    # Optional inbox scoping so callers (e.g. the WhatsApp call button) can
-    # find the latest open conversation in a specific inbox even when the
-    # 20-row cap below would otherwise cut it off.
-    conversations = conversations.where(inbox_id: params[:inbox_id]) if params[:inbox_id].present?
 
     # Apply permission-based filtering using the existing service
     conversations = Conversations::PermissionFilterService.new(