fix: 恢复 GetGenerationTaskUseCase 导出

Phase 1 清理时误删了 packages/application/__init__.py 中的
GetGenerationTaskUseCase 导出，导致 generation_tasks.py 路由
ImportError。

packages/application/__init__.py

@@ -27,6 +27,7 @@ from .generated_videos import (
 from .generation_tasks import (
     CreateGenerationTaskCommand,
     CreateGenerationTaskUseCase,
+    GetGenerationTaskUseCase,
 )
 from .ingest_jobs import SubmitIngestJobCommand, SubmitIngestJobUseCase
 from .jobs import (
@@ -63,6 +64,7 @@ __all__ = [
     "CreateAssetUseCase",
     "CreateGenerationTaskCommand",
     "CreateGenerationTaskUseCase",
+    "GetGenerationTaskUseCase",
     "CreateJobCommand",
     "CreateJobUseCase",
     "CreateProjectCommand",