Implement multiple phone numbers per workflow feature

Database Changes:
- Added twp_workflow_phones junction table for many-to-many relationship
- Updated activator to create and manage new table
- Maintained backward compatibility with existing phone_number field

Workflow Management:
- Added set_workflow_phone_numbers() and get_workflow_phone_numbers() methods
- Updated get_workflow_by_phone_number() to check both old and new structures
- Enhanced save/update handlers to support phone_numbers array
- Added AJAX endpoint for retrieving workflow phone numbers

User Interface:
- Replaced single phone number select with dynamic multi-select interface
- Added "Add Number" and "Remove" buttons for managing multiple numbers
- Updated workflow listing to display all assigned phone numbers
- Enhanced JavaScript for phone number management and validation

The webhook routing already supports multiple numbers since get_workflow_by_phone_number()
was updated to check both structures. This feature allows users to assign multiple
Twilio phone numbers to trigger the same workflow.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

includes/class-twp-core.php

@@ -127,6 +127,7 @@ class TWP_Core {
         $this->loader->add_action('wp_ajax_twp_save_workflow', $plugin_admin, 'ajax_save_workflow');
         $this->loader->add_action('wp_ajax_twp_update_workflow', $plugin_admin, 'ajax_save_workflow');
         $this->loader->add_action('wp_ajax_twp_get_workflow', $plugin_admin, 'ajax_get_workflow');
+        $this->loader->add_action('wp_ajax_twp_get_workflow_phone_numbers', $plugin_admin, 'ajax_get_workflow_phone_numbers');
         $this->loader->add_action('wp_ajax_twp_delete_workflow', $plugin_admin, 'ajax_delete_workflow');
         
         // Phone number management AJAX