Add headless CMS for official site content management

Integrate article and page management into the Laravel admin dashboard
to serve as a headless CMS for the Next.js frontend (usher-site).

Backend:
- 7 migrations: article_categories, article_tags, articles, pivots, attachments, pages
- 5 models with relationships: Article, ArticleCategory, ArticleTag, ArticleAttachment, Page
- 4 admin controllers: articles (with publish/archive/pin), categories, tags, pages
- Admin views with EasyMDE markdown editor, multi-select categories/tags
- Navigation section "官網管理" in admin sidebar

API (v1):
- GET /api/v1/articles (filtered by type, category, tag, search; paginated)
- GET /api/v1/articles/{slug} (with related articles)
- GET /api/v1/categories
- GET /api/v1/pages/{slug} (with children)
- GET /api/v1/homepage (aggregated homepage data)
- Attachment download endpoint
- CORS configured for usher.org.tw, vercel.app, localhost:3000

Content migration:
- ImportHugoContent command: imports Hugo markdown files as articles/pages
- Successfully imported 27 articles, 17 categories, 11 tags, 9 pages

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Http/Resources/CategoryResource.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace App\Http\Resources;
+
+use Illuminate\Http\Request;
+use Illuminate\Http\Resources\Json\JsonResource;
+
+class CategoryResource extends JsonResource
+{
+    public function toArray(Request $request): array
+    {
+        return [
+            'id' => $this->id,
+            'name' => $this->name,
+            'slug' => $this->slug,
+            'description' => $this->description,
+            'articles_count' => $this->whenCounted('articles'),
+        ];
+    }
+}