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-activator.php

@@ -37,6 +37,7 @@ class TWP_Activator {
             'twp_call_queues',
             'twp_queued_calls',
             'twp_workflows',
+            'twp_workflow_phones',
             'twp_call_log',
             'twp_sms_log',
             'twp_voicemails',
@@ -137,7 +138,7 @@ class TWP_Activator {
             KEY status (status)
         ) $charset_collate;";
         
-        // Workflows table
+        // Workflows table (keeping phone_number for backward compatibility, will be deprecated)
         $table_workflows = $wpdb->prefix . 'twp_workflows';
         $sql_workflows = "CREATE TABLE $table_workflows (
             id int(11) NOT NULL AUTO_INCREMENT,
@@ -151,6 +152,19 @@ class TWP_Activator {
             KEY phone_number (phone_number)
         ) $charset_collate;";
         
+        // Workflow phone numbers junction table
+        $table_workflow_phones = $wpdb->prefix . 'twp_workflow_phones';
+        $sql_workflow_phones = "CREATE TABLE $table_workflow_phones (
+            id int(11) NOT NULL AUTO_INCREMENT,
+            workflow_id int(11) NOT NULL,
+            phone_number varchar(20) NOT NULL,
+            created_at datetime DEFAULT CURRENT_TIMESTAMP,
+            PRIMARY KEY (id),
+            UNIQUE KEY workflow_phone (workflow_id, phone_number),
+            KEY workflow_id (workflow_id),
+            KEY phone_number (phone_number)
+        ) $charset_collate;";
+        
         // Call log table
         $table_call_log = $wpdb->prefix . 'twp_call_log';
         $sql_call_log = "CREATE TABLE $table_call_log (
@@ -274,6 +288,7 @@ class TWP_Activator {
         dbDelta($sql_queues);
         dbDelta($sql_queued_calls);
         dbDelta($sql_workflows);
+        dbDelta($sql_workflow_phones);
         dbDelta($sql_call_log);
         dbDelta($sql_sms_log);
         dbDelta($sql_voicemails);