feat(updates): wire PUC v5 registration into Plugin::boot()

register_update_checker() guards on is_file + class_exists so the plugin
degrades cleanly on installs that ship without the vendored lib (no update
notices, but plugin core remains functional).

includes/class-plugin.php

@@ -50,10 +50,30 @@ final class Plugin
         Dispatcher::register_hooks();
         (new CF7Source())->register();
 
+        self::register_update_checker();
+
         // Admin
         if (is_admin()) {
             (new Settings_Page())->register();
             (new Dashboard_Widget())->register();
         }
     }
+
+    private static function register_update_checker(): void
+    {
+        $loader = CF7STG_PLUGIN_DIR . 'includes/lib/plugin-update-checker/plugin-update-checker.php';
+        if (!is_file($loader)) {
+            return;
+        }
+        require_once $loader;
+        if (!class_exists('YahnisElsts\\PluginUpdateChecker\\v5\\PucFactory')) {
+            return;
+        }
+
+        \YahnisElsts\PluginUpdateChecker\v5\PucFactory::buildUpdateChecker(
+            'https://git.netranking.ru/bryzgalov/cf7-spam-to-telegram/raw/branch/main/wp-plugin-info.json',
+            CF7STG_PLUGIN_FILE,
+            'cf7-spam-to-telegram'
+        );
+    }
 }

tests/UpdateCheckerIntegrationTest.php

@@ -64,4 +64,15 @@ final class UpdateCheckerIntegrationTest extends Cf7stgTestCase
             "download_url must contain expected asset name $expectedAssetName"
         );
     }
+
+    public function test_plugin_class_has_register_update_checker_method(): void
+    {
+        self::assertTrue(
+            method_exists('\\Cf7stg\\Plugin', 'register_update_checker'),
+            'Plugin class must expose register_update_checker()'
+        );
+        $ref = new \ReflectionMethod('\\Cf7stg\\Plugin', 'register_update_checker');
+        self::assertTrue($ref->isStatic(), 'register_update_checker must be static');
+        self::assertTrue($ref->isPrivate(), 'register_update_checker must be private');
+    }
 }