feat: P2 素材库自动匹配 - 支持 all/random/smart 三种素材选取模式

- Schema: 新增 asset_select_mode + asset_select_count 字段
- Domain: GenerationTask 新增 asset_select_mode 字段
- Application: Command/UseCase 透传 asset_select_mode
- Route: _select_assets_from_library() 辅助函数 + 创建任务集成
- SQLAlchemy: Model/Repository 映射 asset_select_mode
- Alembic: 032 号迁移
- Worker: _download_library_assets() 支持 asset_ids 过滤
- 单测: 15 个测试覆盖三种模式 + 边界情况

packages/adapters/sqlalchemy_impl/generation_task_repository.py

@@ -25,6 +25,7 @@ def _to_domain(model: GenerationTaskModel) -> GenerationTask:
         completed_at=model.completed_at,
         created_by_user_id=model.created_by_user_id,
         source_edit_plan_id=model.source_edit_plan_id or "",
+        asset_select_mode=model.asset_select_mode or "",
         created_at=model.created_at,
     )
 
@@ -52,6 +53,7 @@ class SQLAlchemyGenerationTaskRepository:
             completed_at=task.completed_at,
             created_by_user_id=task.created_by_user_id,
             source_edit_plan_id=task.source_edit_plan_id or None,
+            asset_select_mode=task.asset_select_mode or "",
             created_at=task.created_at,
         )
         self.session.add(model)
@@ -123,5 +125,6 @@ class SQLAlchemyGenerationTaskRepository:
         model.started_at = task.started_at
         model.completed_at = task.completed_at
         model.source_edit_plan_id = task.source_edit_plan_id or None
+        model.asset_select_mode = task.asset_select_mode or ""
         self.session.commit()
         return task