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/Models/ArticleAttachment.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+
+class ArticleAttachment extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'article_id',
+        'file_path',
+        'original_filename',
+        'mime_type',
+        'file_size',
+        'description',
+        'download_count',
+    ];
+
+    protected $casts = [
+        'file_size' => 'integer',
+        'download_count' => 'integer',
+    ];
+
+    public function article()
+    {
+        return $this->belongsTo(Article::class);
+    }
+
+    public function incrementDownloadCount(): void
+    {
+        $this->increment('download_count');
+    }
+}