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/Admin/Schema/SchemaPermissions.php

@@ -144,12 +144,6 @@ class SchemaPermissions {
                         ['key' => 'basic', 'title' => '审核总开关', 'fields' => [
                             ['key' => 'enabled', 'label' => '启用审核系统', 'type' => 'switch', 'default' => false],
                         ]],
-                        ['key' => 'akismet', 'title' => 'Akismet', 'fields' => [
-                            ['key' => 'akismet_enabled', 'label' => '启用 Akismet', 'type' => 'switch', 'default' => false, 'dependsOn' => ['key' => 'enabled', 'value' => true]],
-                            ['key' => 'akismet_site_url', 'label' => 'Site URL', 'type' => 'input', 'default' => '', 'dependsOn' => ['key' => 'akismet_enabled', 'value' => true]],
-                            ['key' => 'akismet_blog_lang', 'label' => '博客语言', 'type' => 'input', 'default' => 'zh-CN', 'dependsOn' => ['key' => 'akismet_enabled', 'value' => true]],
-                            ['key' => 'akismet_api_key', 'label' => 'API Key', 'type' => 'input', 'inputType' => 'password', 'default' => '', 'dependsOn' => ['key' => 'akismet_enabled', 'value' => true]],
-                        ]],
                         ['key' => 'ai', 'title' => 'AI 审核', 'fields' => [
                             ['key' => 'ai_enabled', 'label' => '启用 AI 审核', 'type' => 'switch', 'default' => false, 'dependsOn' => ['key' => 'enabled', 'value' => true]],
                             ['key' => 'ai_api_hint', 'label' => '审核 API 配置', 'type' => 'notice', 'alertType' => 'info', 'alertContent' => 'API 接入在「扩展 → API 内容审核」中统一配置；此处仅控制审核范围和策略。', 'dependsOn' => ['key' => 'ai_enabled', 'value' => true]],