feat: R2 presigned URL 엔드포인트 추가

- FileStorageController에 presignedUrl() 메서드 추가 (30분 유효)
- GET /api/v1/files/{id}/presigned-url 라우트 추가
- 파일 프록시 스트리밍 대신 R2 직접 접근 지원

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/Http/Controllers/Api/V1/FileStorageController.php

@@ -9,6 +9,7 @@
 use App\Http\Requests\Api\V1\ShareLinkRequest;
 use App\Services\FileStorageService;
 use Illuminate\Http\Request;
+use Illuminate\Support\Facades\Storage;
 
 class FileStorageController extends Controller
 {
@@ -122,6 +123,28 @@ public function view(int $id, Request $request)
         return $file->download(inline: true);
     }
 
+    /**
+     * R2 Presigned URL 발급 (30분 유효)
+     */
+    public function presignedUrl(int $id, Request $request)
+    {
+        $this->ensureContext($request);
+
+        $service = new FileStorageService;
+        $file = $service->getFile($id);
+
+        if (! $file->file_path || ! Storage::disk('r2')->exists($file->file_path)) {
+            abort(404, 'File not found in storage');
+        }
+
+        $url = Storage::disk('r2')->temporaryUrl(
+            $file->file_path,
+            now()->addMinutes(30)
+        );
+
+        return ApiResponse::handle(fn () => ['url' => $url]);
+    }
+
     /**
      * Soft delete file
      */