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
      *

src/app/Models/DocumentTranslation.php

@@ -0,0 +1,72 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use League\CommonMark\CommonMarkConverter;
+use League\CommonMark\Extension\GithubFlavoredMarkdownExtension;
+
+class DocumentTranslation extends Model
+{
+    use HasFactory;
+
+    protected $fillable = [
+        'document_id',
+        'locale',
+        'title',
+        'content',
+        'rendered_html',
+        'created_by',
+        'updated_by',
+    ];
+
+    public function document(): BelongsTo
+    {
+        return $this->belongsTo(Document::class);
+    }
+
+    public function creator(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'created_by');
+    }
+
+    public function updater(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'updated_by');
+    }
+
+    /**
+     * Full-text search scope. Falls back to LIKE on non-MySQL drivers
+     * (notably SQLite in tests, which lacks FULLTEXT).
+     */
+    public function scopeSearch(Builder $query, string $term): Builder
+    {
+        if ($query->getConnection()->getDriverName() === 'mysql') {
+            return $query->whereRaw(
+                'MATCH(title, content) AGAINST(? IN BOOLEAN MODE)',
+                [$term]
+            );
+        }
+
+        return $query->where(function (Builder $q) use ($term) {
+            $like = '%' . $term . '%';
+            $q->where('title', 'like', $like)->orWhere('content', 'like', $like);
+        });
+    }
+
+    public static function renderMarkdown(string $markdown): string
+    {
+        $converter = new CommonMarkConverter([
+            'html_input' => 'strip',
+            'allow_unsafe_links' => false,
+        ]);
+
+        $converter->getEnvironment()->addExtension(new GithubFlavoredMarkdownExtension());
+        $converter->getEnvironment()->addExtension(new \App\Markdown\MediaEmbedExtension());
+
+        return $converter->convert($markdown)->getContent();
+    }
+}

src/tests/Unit/Models/DocumentTranslationTest.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace Tests\Unit\Models;
+
+use App\Models\Document;
+use App\Models\DocumentTranslation;
+use Illuminate\Database\QueryException;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Tests\TestCase;
+
+class DocumentTranslationTest extends TestCase
+{
+    use RefreshDatabase;
+
+    public function test_belongs_to_a_document(): void
+    {
+        $doc = Document::factory()->create();
+        $translation = $doc->translations()->first();
+
+        $this->assertInstanceOf(Document::class, $translation->document);
+        $this->assertSame($doc->id, $translation->document->id);
+    }
+
+    public function test_unique_document_locale_constraint(): void
+    {
+        $doc = Document::factory()->create(['default_locale' => 'en']);
+
+        $this->expectException(QueryException::class);
+
+        DocumentTranslation::create([
+            'document_id' => $doc->id,
+            'locale' => 'en',
+            'title' => 'Duplicate',
+            'content' => 'x',
+            'rendered_html' => '<p>x</p>',
+        ]);
+    }
+
+    public function test_cascade_delete_when_document_deleted(): void
+    {
+        $doc = Document::factory()->create();
+        $translationId = $doc->translations()->first()->id;
+
+        $doc->forceDelete();
+
+        $this->assertNull(DocumentTranslation::find($translationId));
+    }
+
+    public function test_render_markdown_converts_basic_markdown(): void
+    {
+        $html = DocumentTranslation::renderMarkdown('# Hello');
+        $this->assertStringContainsString('<h1>', $html);
+        $this->assertStringContainsString('Hello', $html);
+    }
+}