feat(persistence): add WorkspaceMembershipRow ORM model

新建 backend/packages/harness/deerflow/persistence/workspace_membership/
{__init__.py, model.py}，注册到 Base.metadata。

WorkspaceMembershipRow schema（来自 workspace-schema-design §2.2 锁定版）：
  - workspace_id String(36) FK workspaces.id ON DELETE CASCADE — PK part
  - user_id String(36) FK users.id ON DELETE CASCADE — PK part
  - role String(16) NOT NULL（Stage 0 仅写 'owner'，schema 允许 'admin'/'member'
    为 Stage 2 RBAC 准备）
  - invited_by String(36) FK users.id ON DELETE SET NULL nullable（Stage 2
    invitation 流程用）
  - joined_at DateTime(tz=True)

索引：
  - idx_workspace_memberships_user (user_id, workspace_id) — 倒查索引让
    /auth/me 列 user 所属 workspaces 走索引
  - idx_one_owner_per_workspace partial UNIQUE on workspace_id WHERE role='owner'
    —— 一 workspace 严格 1 个 owner；sqlite_where + postgresql_where 双驱动
    并存（实测两边都识别）

Stage 0 PR3 T3.6（提前到 T3.4-T3.5 之前；WorkspaceRepository.list_by_user
等会 JOIN 这张表）。

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

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

@@ -9,6 +9,7 @@ The actual ORM classes have moved to entity-specific subpackages:
 - ``deerflow.persistence.feedback``
 - ``deerflow.persistence.user``
 - ``deerflow.persistence.workspace``  (Stage 0 PR3)
+- ``deerflow.persistence.workspace_membership``  (Stage 0 PR3)
 
 ``RunEventRow`` remains in ``deerflow.persistence.models.run_event`` because
 its storage implementation lives in ``deerflow.runtime.events.store.db`` and
@@ -21,5 +22,14 @@ 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
+from deerflow.persistence.workspace_membership.model import WorkspaceMembershipRow
 
-__all__ = ["FeedbackRow", "RunEventRow", "RunRow", "ThreadMetaRow", "UserRow", "WorkspaceRow"]
+__all__ = [
+    "FeedbackRow",
+    "RunEventRow",
+    "RunRow",
+    "ThreadMetaRow",
+    "UserRow",
+    "WorkspaceMembershipRow",
+    "WorkspaceRow",
+]

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

@@ -0,0 +1,16 @@
+"""Workspace membership persistence — ORM model + repository.
+
+Tracks who is a member of which workspace and what their role is
+within that workspace. Composite primary key ``(workspace_id, user_id)``;
+``role`` follows the three-state ``owner`` / ``admin`` / ``member`` model
+(Stage 0 only writes ``'owner'``; Stage 2 RBAC rollout opens admin/member).
+
+A workspace MUST have exactly one ``owner`` — enforced by a partial
+unique index. Owner transfer is a two-row transactional swap.
+"""
+
+from __future__ import annotations
+
+from deerflow.persistence.workspace_membership.model import WorkspaceMembershipRow
+
+__all__ = ["WorkspaceMembershipRow"]

backend/packages/harness/deerflow/persistence/workspace_membership/model.py

@@ -0,0 +1,62 @@
+"""ORM model for workspace memberships (which user is in which workspace, with what role)."""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+from sqlalchemy import DateTime, ForeignKey, Index, String, text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from deerflow.persistence.base import Base
+
+
+class WorkspaceMembershipRow(Base):
+    __tablename__ = "workspace_memberships"
+
+    workspace_id: Mapped[str] = mapped_column(
+        String(36),
+        ForeignKey("workspaces.id", ondelete="CASCADE"),
+        primary_key=True,
+        comment="所属 workspace；workspace 删除时级联清掉成员记录",
+    )
+    user_id: Mapped[str] = mapped_column(
+        String(36),
+        ForeignKey("users.id", ondelete="CASCADE"),
+        primary_key=True,
+        comment="成员 user_id；用户删除时级联清掉成员记录",
+    )
+    role: Mapped[str] = mapped_column(
+        String(16),
+        nullable=False,
+        comment='角色字符串：Stage 0 仅写 "owner"；Stage 2 RBAC 打开 "admin"/"member"。用 String(16) 而非 enum 以便未来加 "viewer"/"auditor" 不动 schema',
+    )
+    invited_by: Mapped[str | None] = mapped_column(
+        String(36),
+        ForeignKey("users.id", ondelete="SET NULL"),
+        nullable=True,
+        comment="邀请人 user_id（Stage 2 invitation 流程才写）；邀请人被删时此字段清空（不影响成员记录本身）",
+    )
+    joined_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        nullable=False,
+        default=lambda: datetime.now(UTC),
+        comment="加入 workspace 的时间（UTC）",
+    )
+
+    __table_args__ = (
+        # 倒查索引：列出 user 所属的所有 workspace（/auth/me 用）。
+        # 复合主键 (workspace_id, user_id) 的前导列是 workspace_id，
+        # 所以"按 user_id 查"需要单独的索引。
+        Index("idx_workspace_memberships_user", "user_id", "workspace_id"),
+        # 一个 workspace 严格 1 个 owner —— partial unique on role='owner'。
+        # SQLite + Postgres 都支持 WHERE 子句的 partial unique；双驱动
+        # 维护两套等价 where 表达式。
+        Index(
+            "idx_one_owner_per_workspace",
+            "workspace_id",
+            unique=True,
+            sqlite_where=text("role = 'owner'"),
+            postgresql_where=text("role = 'owner'"),
+        ),
+        {"comment": ("工作空间成员表（multi-tenant RBAC）。复合 PK (workspace_id, user_id)；每个 workspace 必有恰好 1 个 owner（partial unique 约束保证）。Stage 0 仅写 owner；Stage 2 RBAC 打开 admin/member。")},
+    )