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/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）