Add SDK persistence and configurable edge location

- Add external SDK installation (wp-content/twilio-sdk/) that survives
  WordPress plugin updates
- Add install-twilio-sdk-external.sh script for external SDK setup
- Update SDK loading to check external location first, internal fallback
- Add post-update detection hook to warn if SDK was deleted
- Add configurable Twilio Edge Location setting (default: roaming)
- Fix US calls failing due to hardcoded Sydney edge location

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

includes/class-twp-webhooks.php

@@ -9,9 +9,25 @@ class TWP_Webhooks {
      */
     public function __construct() {
         // Load Twilio SDK if not already loaded
+        // Check external location first (survives plugin updates), then internal
         if (!class_exists('\Twilio\Rest\Client')) {
-            $autoloader_path = plugin_dir_path(dirname(__FILE__)) . 'vendor/autoload.php';
-            if (file_exists($autoloader_path)) {
+            $autoloader_path = null;
+
+            // Priority 1: External SDK location
+            $external_autoloader = dirname(dirname(plugin_dir_path(dirname(__FILE__)))) . '/twilio-sdk/autoload.php';
+            if (file_exists($external_autoloader)) {
+                $autoloader_path = $external_autoloader;
+            }
+
+            // Priority 2: Internal vendor directory
+            if (!$autoloader_path) {
+                $internal_autoloader = plugin_dir_path(dirname(__FILE__)) . 'vendor/autoload.php';
+                if (file_exists($internal_autoloader)) {
+                    $autoloader_path = $internal_autoloader;
+                }
+            }
+
+            if ($autoloader_path) {
                 require_once $autoloader_path;
             }
         }