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>

assets/admin/src/schema/sections/permissions/index.js

@@ -130,10 +130,6 @@ export const permissionsSchemas = {
     description: '管理评论审核、反垃圾、限流与拦截配置。',
     defaults: {
       enabled: false,
-      akismet_enabled: false,
-      akismet_site_url: '',
-      akismet_blog_lang: 'zh-CN',
-      akismet_api_key: '',
       ai_enabled: false,
       ai_moderate_comments: true,
       ai_moderate_background: true,
@@ -157,17 +153,6 @@ export const permissionsSchemas = {
           { key: 'enabled', label: '启用审核系统', type: 'switch', help: '关闭后评论将不经过主题审核链路。' },
         ],
       },
-      {
-        key: 'akismet',
-        title: 'Akismet',
-        icon: 'fa fas fa-filter',
-        fields: [
-          { key: 'akismet_enabled', label: '启用 Akismet', type: 'switch', dependsOn: { key: 'enabled', value: true } },
-          { key: 'akismet_site_url', label: 'Site URL', type: 'input', placeholder: 'https://your-site.com', dependsOn: { key: 'akismet_enabled', value: true } },
-          { key: 'akismet_blog_lang', label: '博客语言', type: 'input', placeholder: 'zh-CN', dependsOn: { key: 'akismet_enabled', value: true } },
-          { key: 'akismet_api_key', label: 'API Key', type: 'input', inputType: 'password', placeholder: 'Akismet API Key', dependsOn: { key: 'akismet_enabled', value: true } },
-        ],
-      },
       {
         key: 'ai',
         title: 'AI 审核',