Add translation CRUD routes and controller

POST/DELETE for translations gated by can:update,document middleware.
Locale validated against SUPPORTED_LOCALES. Default-locale deletion
returns 422; duplicate-locale add returns 422. Flash messages added
to en/ja lang files (other locales updated in Task 9).

src/app/Http/Controllers/DocumentTranslationController.php

@@ -0,0 +1,54 @@
+<?php
+
+namespace App\Http\Controllers;
+
+use App\Http\Middleware\SetLocale;
+use App\Models\Document;
+use App\Services\DocumentService;
+use Illuminate\Http\Request;
+use Illuminate\Support\Facades\Auth;
+
+class DocumentTranslationController extends Controller
+{
+    public function __construct(private DocumentService $service) {}
+
+    public function store(Request $request, Document $document)
+    {
+        $validated = $request->validate([
+            'locale' => ['required', 'string', 'in:' . implode(',', array_keys(SetLocale::SUPPORTED_LOCALES))],
+            'title' => ['required', 'string', 'max:255'],
+            'content' => ['required', 'string'],
+        ]);
+
+        try {
+            $this->service->addTranslation(
+                $document,
+                $validated['locale'],
+                $validated['title'],
+                $validated['content'],
+                Auth::id(),
+            );
+        } catch (\InvalidArgumentException $e) {
+            return response()->json(['message' => $e->getMessage()], 422);
+        }
+
+        return redirect()->route('documents.show', $document)
+            ->with('message', __('messages.documents.translation_added'));
+    }
+
+    public function destroy(Document $document, string $locale)
+    {
+        if (!array_key_exists($locale, SetLocale::SUPPORTED_LOCALES)) {
+            abort(404);
+        }
+
+        try {
+            $this->service->deleteTranslation($document, $locale);
+        } catch (\InvalidArgumentException $e) {
+            return response()->json(['message' => $e->getMessage()], 422);
+        }
+
+        return redirect()->route('documents.show', $document)
+            ->with('message', __('messages.documents.translation_deleted'));
+    }
+}

src/lang/en/messages.php

@@ -39,6 +39,8 @@
         'content_label' => 'Content',
         'content_placeholder' => 'Write your markdown here...',
         'saving' => 'Saving...',
+        'translation_added' => 'Translation added.',
+        'translation_deleted' => 'Translation deleted.',
     ],
 
     // Quick Switcher

src/lang/ja/messages.php

@@ -39,6 +39,8 @@
         'content_label' => '本文',
         'content_placeholder' => 'Markdownで記述してください...',
         'saving' => '保存中...',
+        'translation_added' => '翻訳を追加しました。',
+        'translation_deleted' => '翻訳を削除しました。',
     ],
 
     // Quick Switcher

src/routes/web.php

@@ -47,6 +47,15 @@
         Route::get('/{document}/edit', DocumentEditor::class)
             ->middleware('can:update,document')
             ->name('edit');
+        Route::post('/{document}/translations', [\App\Http\Controllers\DocumentTranslationController::class, 'store'])
+            ->middleware('can:update,document')
+            ->name('translations.store');
+        Route::delete('/{document}/translations/{locale}', [\App\Http\Controllers\DocumentTranslationController::class, 'destroy'])
+            ->middleware('can:update,document')
+            ->name('translations.destroy');
+        Route::get('/{document}/translations/{locale}/edit', \App\Livewire\DocumentEditor::class)
+            ->middleware('can:update,document')
+            ->name('translations.edit');
     });
 
     // 公開ルート（動的ルートは最後に）

src/tests/Feature/DocumentTranslationCrudTest.php

@@ -0,0 +1,94 @@
+<?php
+
+namespace Tests\Feature;
+
+use App\Models\Document;
+use App\Models\User;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Tests\TestCase;
+
+class DocumentTranslationCrudTest extends TestCase
+{
+    use RefreshDatabase;
+
+    public function test_owner_can_add_a_translation(): void
+    {
+        $owner = User::factory()->create();
+        $doc = Document::factory()->create(['default_locale' => 'en', 'created_by' => $owner->id]);
+
+        $response = $this->actingAs($owner)->post(
+            route('documents.translations.store', $doc),
+            ['locale' => 'ja', 'title' => 'こんにちは', 'content' => '本文']
+        );
+
+        $response->assertRedirect();
+        $this->assertNotNull($doc->fresh()->translationFor('ja', false));
+    }
+
+    public function test_non_owner_cannot_add_translation(): void
+    {
+        $owner = User::factory()->create();
+        $other = User::factory()->create();
+        $doc = Document::factory()->create(['default_locale' => 'en', 'created_by' => $owner->id]);
+
+        $response = $this->actingAs($other)->post(
+            route('documents.translations.store', $doc),
+            ['locale' => 'ja', 'title' => 'こんにちは', 'content' => '本文']
+        );
+
+        $response->assertForbidden();
+    }
+
+    public function test_invalid_locale_is_rejected(): void
+    {
+        $owner = User::factory()->create();
+        $doc = Document::factory()->create(['default_locale' => 'en', 'created_by' => $owner->id]);
+
+        $response = $this->actingAs($owner)->post(
+            route('documents.translations.store', $doc),
+            ['locale' => 'xx', 'title' => 'X', 'content' => 'Y']
+        );
+
+        $response->assertSessionHasErrors('locale');
+    }
+
+    public function test_duplicate_locale_returns_422(): void
+    {
+        $owner = User::factory()->create();
+        $doc = Document::factory()->create(['default_locale' => 'en', 'created_by' => $owner->id]);
+
+        $response = $this->actingAs($owner)->post(
+            route('documents.translations.store', $doc),
+            ['locale' => 'en', 'title' => 'X', 'content' => 'Y']
+        );
+
+        $response->assertStatus(422);
+    }
+
+    public function test_owner_can_delete_non_default_translation(): void
+    {
+        $owner = User::factory()->create();
+        $doc = Document::factory()->create(['default_locale' => 'en', 'created_by' => $owner->id]);
+        app(\App\Services\DocumentService::class)->addTranslation($doc, 'ja', 'JA', 'body', $owner->id);
+
+        $response = $this->actingAs($owner)->delete(
+            route('documents.translations.destroy', ['document' => $doc, 'locale' => 'ja'])
+        );
+
+        $response->assertRedirect();
+        $this->assertNull($doc->fresh()->translationFor('ja', false));
+    }
+
+    public function test_default_locale_translation_cannot_be_deleted(): void
+    {
+        $owner = User::factory()->create();
+        $doc = Document::factory()->create(['default_locale' => 'en', 'created_by' => $owner->id]);
+
+        $response = $this->actingAs($owner)->delete(
+            route('documents.translations.destroy', ['document' => $doc, 'locale' => 'en'])
+        );
+
+        $response->assertStatus(422);
+        $this->assertNotNull($doc->fresh()->translationFor('en', false));
+    }
+}