fix: generation_tasks results 端点返回 OSS 预签名 URL（24h 有效期）

P0-2 修复：list_generation_results 端点原先直接返回原始 file_url，
OSS bucket 为 private 导致前端访问视频返回 403。

改动：
- 注入 storage_service 依赖（复用已有 OSSStorageService.get_download_url）
- 对每个视频的 file_url 生成预签名 URL，有效期 86400 秒（24h）
- _to_generated_video_response 支持 download_url 参数
- 与 generated_videos.py 中的模式保持一致

新增测试：
- test_results_endpoint_generates_presigned_urls：验证预签名 URL 生成
- test_results_endpoint_handles_empty_videos：验证空结果正常返回

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/api/app/api/routes/generation_tasks.py

@@ -4,6 +4,7 @@ from typing import Any
 
 from app.auth import AuthenticatedUser, get_current_user
 from app.core.celery_app import celery_app
+from app.core.storage import OSSStorageService, get_storage_service
 from app.dependencies import (
     get_asset_library_repository,
     get_asset_repository,
@@ -63,7 +64,7 @@ def _to_generation_task_response(task) -> GenerationTaskResponse:
     )
 
 
-def _to_generated_video_response(item) -> GeneratedVideoResponse:
+def _to_generated_video_response(item, download_url: str | None = None) -> GeneratedVideoResponse:
     return GeneratedVideoResponse(
         id=item.id,
         project_id=item.project_id,
@@ -76,6 +77,7 @@ def _to_generated_video_response(item) -> GeneratedVideoResponse:
         width=item.width,
         height=item.height,
         fps=item.fps,
+        download_url=download_url,
     )
 
 
@@ -265,6 +267,7 @@ def list_generation_results(
     generation_task_repository: Any = Depends(get_generation_task_repository),
     generated_video_repository: Any = Depends(get_generated_video_repository),
     project_repository: Any = Depends(get_project_repository),
+    storage_service: OSSStorageService = Depends(get_storage_service),
 ) -> ListGeneratedVideosResponse:
     task = generation_task_repository.get(task_id)
     if task is None:
@@ -273,7 +276,11 @@ def list_generation_results(
         _check_project_access(task.project_id, authenticated_user.user.id, project_repository)
     use_case = ListGeneratedVideosByTaskUseCase(generated_video_repository)
     items = use_case.execute(task_id)
-    return ListGeneratedVideosResponse(items=[_to_generated_video_response(item) for item in items])
+    responses = []
+    for item in items:
+        download_url = storage_service.get_download_url(item.file_url, expires_seconds=86400)
+        responses.append(_to_generated_video_response(item, download_url=download_url))
+    return ListGeneratedVideosResponse(items=responses)
 
 
 @router.post("/tasks/{task_id}/retry", response_model=GenerationTaskResponse)