feat(persistence): alembic 0001 — users.default_workspace_id

First Alembic revision for the DeerFlow application schema. Adds a
nullable users.default_workspace_id column with an ON DELETE SET NULL
FK to workspaces(id), so newly-registered users can be routed back to
their default workspace on next login without consulting memberships.

Uses op.batch_alter_table for SQLite ALTER compatibility (sqlite is
still a supported dev fallback).

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

backend/packages/harness/deerflow/persistence/migrations/versions/0001_users_default_workspace.py

@@ -0,0 +1,44 @@
+"""users.default_workspace_id column + FK to workspaces
+
+Revision ID: 0001_users_default_workspace
+Revises: None
+Create Date: 2026-05-12
+
+First Alembic revision for the DeerFlow application schema. Adds
+`users.default_workspace_id` so a newly-registered user can be sent
+back to their default workspace on next login without consulting the
+memberships table.
+
+Existing deployments created `users` via `metadata.create_all()` without
+this column; running `alembic upgrade head` on those DBs will simply add
+the column (ON DELETE SET NULL FK), no data backfill needed.
+"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+# Alembic identifiers.
+revision: str = "0001_users_default_workspace"
+down_revision: str | None = None
+branch_labels: str | None = None
+depends_on: str | None = None
+
+
+def upgrade() -> None:
+    with op.batch_alter_table("users") as batch:
+        batch.add_column(sa.Column("default_workspace_id", sa.String(36), nullable=True))
+        batch.create_foreign_key(
+            "fk_users_default_workspace",
+            "workspaces",
+            ["default_workspace_id"],
+            ["id"],
+            ondelete="SET NULL",
+        )
+
+
+def downgrade() -> None:
+    with op.batch_alter_table("users") as batch:
+        batch.drop_constraint("fk_users_default_workspace", type_="foreignkey")
+        batch.drop_column("default_workspace_id")