feat(persistence): alembic 0002 — business tables nullable workspace_id

ALTER threads_meta / runs / feedback / run_events to add nullable
workspace_id String(36) + FK to workspaces (ON DELETE CASCADE) and a
composite (workspace_id, user_id, updated_at) index on threads_meta for
the workspace-scoped thread-list query. Downgrade reverses all four FKs,
columns, and the index.

Stage 0 PR5 step 1/2; revision 0003 flips NOT NULL after the backfill.

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

backend/packages/harness/deerflow/persistence/migrations/versions/0002_business_tables_workspace.py

@@ -0,0 +1,70 @@
+"""Business tables: nullable workspace_id + FK to workspaces
+
+Revision ID: 0002_business_tables_workspace
+Revises: 0001_users_default_workspace
+Create Date: 2026-05-13
+
+Stage 0 PR5 step 1/2 (the second step lives in revision 0003).
+
+This revision adds a *nullable* ``workspace_id`` column to the four
+business tables that need tenancy scoping:
+
+  * ``threads_meta``
+  * ``runs``
+  * ``feedback``
+  * ``run_events``
+
+The column is nullable here on purpose — running ``upgrade`` on a
+database with existing rows leaves those rows with ``workspace_id = NULL``
+until ``scripts/backfill_workspace_id.py`` populates them. Once the
+backfill finishes, revision 0003 flips the column to ``NOT NULL`` and
+adds the threads_meta ``(workspace_id, thread_id)`` UNIQUE index.
+
+A composite index ``idx_threads_meta_workspace_user_updated`` is added
+on ``threads_meta`` to support the common "list a workspace's threads
+for a user, newest first" access pattern that PR6 routers will rely on.
+"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0002_business_tables_workspace"
+down_revision: str | None = "0001_users_default_workspace"
+branch_labels: str | None = None
+depends_on: str | None = None
+
+
+# Tables that receive the new column. The order matters only for human
+# readability in migration logs — there are no inter-table data deps
+# during ALTER, and the FK references workspaces (introduced in PR3) which
+# is already present at this point.
+_BUSINESS_TABLES = ("threads_meta", "runs", "feedback", "run_events")
+
+
+def upgrade() -> None:
+    for table in _BUSINESS_TABLES:
+        with op.batch_alter_table(table) as batch:
+            batch.add_column(sa.Column("workspace_id", sa.String(36), nullable=True))
+            batch.create_foreign_key(
+                f"fk_{table}_workspace_id",
+                "workspaces",
+                ["workspace_id"],
+                ["id"],
+                ondelete="CASCADE",
+            )
+
+    op.create_index(
+        "idx_threads_meta_workspace_user_updated",
+        "threads_meta",
+        ["workspace_id", "user_id", "updated_at"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("idx_threads_meta_workspace_user_updated", table_name="threads_meta")
+    for table in reversed(_BUSINESS_TABLES):
+        with op.batch_alter_table(table) as batch:
+            batch.drop_constraint(f"fk_{table}_workspace_id", type_="foreignkey")
+            batch.drop_column("workspace_id")