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/models.py

@@ -253,6 +253,7 @@ class GenerationTaskModel(Base):
     completed_at = Column(DateTime, nullable=True)
     created_by_user_id = Column(String(32), nullable=False, default="", index=True)
     source_edit_plan_id = Column(String(32), nullable=True, index=True)
+    asset_select_mode = Column(String(20), nullable=False, default="")
     extra_meta = Column("metadata", JSON, nullable=False, default=dict)
     created_at = Column(DateTime, nullable=False, default=lambda: datetime.now(timezone.utc))