feat(persistence): 4 business ORM models — nullable workspace_id

ORM-side mirror of alembic 0002. threads_meta gets the matching
(workspace_id, user_id, updated_at) composite index so fresh
create_all() dev databases land on the same shape Alembic produces.
Pre-existing test_alembic_default_workspace_id pins to revision
0001 explicitly since 'head' now includes 0002.

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

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

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 from datetime import UTC, datetime
 
-from sqlalchemy import JSON, DateTime, String
+from sqlalchemy import JSON, DateTime, ForeignKey, Index, String
 from sqlalchemy.orm import Mapped, mapped_column
 
 from deerflow.persistence.base import Base
@@ -16,10 +16,22 @@ class ThreadMetaRow(Base):
     thread_id: Mapped[str] = mapped_column(String(64), primary_key=True, comment="会话主键（LangGraph thread_id）")
     assistant_id: Mapped[str | None] = mapped_column(String(128), index=True, comment="关联的 Assistant ID（自定义智能体名）；为 NULL 表示默认 lead agent")
     user_id: Mapped[str | None] = mapped_column(String(64), index=True, comment="会话所有者；为 NULL 表示历史无主数据")
+    workspace_id: Mapped[str | None] = mapped_column(
+        String(36),
+        ForeignKey("workspaces.id", ondelete="CASCADE"),
+        nullable=True,
+        comment="所属 workspace；PR5 期间 nullable（回填中），PR5 0003 迁移后改 NOT NULL",
+    )
     display_name: Mapped[str | None] = mapped_column(String(256), comment="会话显示名（自动生成的标题或用户手改）")
     status: Mapped[str] = mapped_column(String(20), default="idle", comment='会话状态："idle" 空闲 / "busy" 正在产出')
     metadata_json: Mapped[dict] = mapped_column(JSON, default=dict, comment="任意扩展元数据（JSON）")
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(UTC), comment="创建时间（UTC）")
     updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(UTC), onupdate=lambda: datetime.now(UTC), comment="最近更新时间（UTC，写入时自动更新）")
 
-    __table_args__ = ({"comment": "会话元数据（每个 LangGraph thread 的概要信息）"},)
+    __table_args__ = (
+        # Workspace-scoped "list threads of this user, newest first" index;
+        # added by alembic 0002. Mirrored on the ORM side so create_all()
+        # produces the same shape on fresh dev databases.
+        Index("idx_threads_meta_workspace_user_updated", "workspace_id", "user_id", "updated_at"),
+        {"comment": "会话元数据（每个 LangGraph thread 的概要信息）"},
+    )