Fix fatal "TWP_Activator not found" on voicemail webhook callback

The activator class was only loaded during plugin activation, on admin_init
when the DB version differed, and in a few explicit admin handlers. Twilio
webhook callbacks (voicemail in particular, but also any non-admin caller of
TWP_Activator::force_table_updates() / ::ensure_tables_exist()) hit the public
REST endpoint where none of those load paths fire, so the static call fataled.

Load class-twp-activator.php in TWP_Core::load_dependencies() so every plugin
context — webhook, REST, cron, admin — has the class available. The activator
file is a pure class definition with no load-time side effects.

This single change covers all 9 runtime call sites (webhook, scheduler, and
six admin sites) that previously assumed the class would already be loaded.

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

includes/class-twp-core.php

@@ -29,7 +29,12 @@ class TWP_Core {
     private function load_dependencies() {
         // Loader class
         require_once TWP_PLUGIN_DIR . 'includes/class-twp-loader.php';
-        
+
+        // Activator (defines TWP_Activator). Loaded here so runtime callers in
+        // webhook/REST/cron contexts can use ::ensure_tables_exist() and
+        // ::force_table_updates() without each site needing its own require_once.
+        require_once TWP_PLUGIN_DIR . 'includes/class-twp-activator.php';
+
         // API classes
         require_once TWP_PLUGIN_DIR . 'includes/class-twp-twilio-api.php';
         require_once TWP_PLUGIN_DIR . 'includes/class-twp-elevenlabs-api.php';