feat(persistence): add WorkspaceRow ORM model

新建 backend/packages/harness/deerflow/persistence/workspace/{__init__.py, model.py}
+ 在 persistence.models.__init__ 注册 WorkspaceRow 让 Base.metadata.create_all
能自动建表。

WorkspaceRow schema（来自 workspace-schema-design §2.1 锁定版）：
  - id String(36) PK（UUID v4 字符串，与 users.id 对齐）
  - name String(64) NOT NULL（显示名）
  - slug String(32) NOT NULL UNIQUE（URL 标识，正则 ^[a-z0-9](-?[a-z0-9])*$）
  - status String(16) default 'active'（active/suspended/deleted）
  - owner_id String(36) FK users.id ON DELETE RESTRICT（删 owner 时阻拦，
    必须先转让所有权）
  - created_at / updated_at DateTime(tz=True)

每列 + 表都带中文 comment（沿用 commit 9ff79055 的 convention）。

Stage 0 PR3 T3.3。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/packages/harness/deerflow/persistence/models/__init__.py

@@ -8,6 +8,7 @@ The actual ORM classes have moved to entity-specific subpackages:
 - ``deerflow.persistence.run``
 - ``deerflow.persistence.feedback``
 - ``deerflow.persistence.user``
+- ``deerflow.persistence.workspace``  (Stage 0 PR3)
 
 ``RunEventRow`` remains in ``deerflow.persistence.models.run_event`` because
 its storage implementation lives in ``deerflow.runtime.events.store.db`` and
@@ -19,5 +20,6 @@ from deerflow.persistence.models.run_event import RunEventRow
 from deerflow.persistence.run.model import RunRow
 from deerflow.persistence.thread_meta.model import ThreadMetaRow
 from deerflow.persistence.user.model import UserRow
+from deerflow.persistence.workspace.model import WorkspaceRow
 
-__all__ = ["FeedbackRow", "RunEventRow", "RunRow", "ThreadMetaRow", "UserRow"]
+__all__ = ["FeedbackRow", "RunEventRow", "RunRow", "ThreadMetaRow", "UserRow", "WorkspaceRow"]