refactor(moderation): remove the Akismet backend and its admin fields

The comment moderation pipeline no longer calls Akismet. Removed:
- check_akismet() and the Akismet step in Moderation::process().
- akismet_* config keys from Moderation defaults and ExtensionRuntime.
- akismet decision state, the jio_moderation_akismet / akismet_result
  comment meta writes, and the legacy-meta fallbacks in the comments column.
- The Akismet fieldset in both the PHP and Vue permissions schemas.

Rate limit, GeoIP, blocked keywords and AI moderation are unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/Settings/ExtensionRuntime.php

@@ -13,10 +13,6 @@ class ExtensionRuntime {
     public static function get_moderation(): array {
         return get_option('jio_moderation_config', [
             'enabled'                  => false,
-            'akismet_enabled'          => false,
-            'akismet_site_url'         => '',
-            'akismet_blog_lang'        => 'zh-CN',
-            'akismet_api_key'          => '',
             'ai_enabled'               => false,
             'ai_provider'              => 'baidu',
             'baidu_api_key'            => '',