Add DocumentTranslation model with renderMarkdown and search scope

Includes a minimal Document::translations() HasMany relation so that
DocumentFactory's afterCreating callback (which calls
$document->translations()->count()) works. The full Document model
refactor (accessors, fallback helpers, default-translation accessor)
lands in Task 4.

src/app/Models/Document.php

@@ -281,6 +281,15 @@ public function recentByUsers(): HasManyThrough
         );
     }
 
+    /**
+     * Translations of this document, one per locale.
+     * (Other relation/accessor refactor lands in Task 4.)
+     */
+    public function translations(): \Illuminate\Database\Eloquent\Relations\HasMany
+    {
+        return $this->hasMany(DocumentTranslation::class);
+    }
+
     /**
      * ディレクトリパスを取得
      *