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/ArticleTag.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Support\Str;
+
+class ArticleTag extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'name',
+        'slug',
+    ];
+
+    protected static function boot()
+    {
+        parent::boot();
+
+        static::creating(function (ArticleTag $tag) {
+            if (empty($tag->slug)) {
+                $slug = Str::slug($tag->name);
+                if (empty($slug)) {
+                    $slug = 'tag-'.time();
+                }
+                $originalSlug = $slug;
+                $count = 1;
+                while (static::where('slug', $slug)->exists()) {
+                    $slug = $originalSlug.'-'.$count++;
+                }
+                $tag->slug = $slug;
+            }
+        });
+    }
+
+    public function articles()
+    {
+        return $this->belongsToMany(Article::class, 'article_tag', 'tag_id', 'article_id');
+    }
+}