feat: post cover filter + admin AI cover action, restyle page-cache admin

- 抽 CoverGenerator 服务（covers:generate 与后台共用），内部强制同步产出 card 转换
- 文章列表：新增封面状态图标列、「有/无封面/完全无图」筛选、行内「生成封面」AI 操作
- 页面缓存：统计改为原生 StatsOverviewWidget（缓存页数/总大小/命中率/时长），
  缓存明细表重排（表头、悬停、URL 截断、图标删除按钮、空状态）

app/Filament/Pages/PageCache.php

@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace App\Filament\Pages;
 
 use App\Blog\Support\PageCacheMiddleware;
+use App\Filament\Widgets\PageCacheStats;
 use App\Models\Setting;
 use Filament\Actions\Action;
 use Filament\Notifications\Notification;
@@ -28,18 +29,7 @@ class PageCache extends Page
     protected string $view = 'filament.pages.page-cache';
 
     /**
-     * @return array{hits: int, misses: int, count: int, rate: float, ttl: int}
-     */
-    public function getStatsProperty(): array
-    {
-        return [
-            ...PageCacheMiddleware::stats(),
-            'ttl' => (int) config('blog.page_cache_ttl', 300),
-        ];
-    }
-
-    /**
-     * @return list<array{key: string, url: string, cached_at: string}>
+     * @return list<array{key: string, url: string, cached_at: string, size: int, expires_at: string}>
      */
     public function getEntriesProperty(): array
     {
@@ -72,6 +62,13 @@ class PageCache extends Page
         Notification::make()->title('已删除这条缓存')->success()->send();
     }
 
+    protected function getHeaderWidgets(): array
+    {
+        return [
+            PageCacheStats::class,
+        ];
+    }
+
     protected function getHeaderActions(): array
     {
         return [