fix(cloud): track workspace membership provenance (#2406)

* fix(cloud): converge legacy revision-zero members

* fix(cloud): track workspace membership provenance

* test(persistence): track current migration head

---------

Co-authored-by: Chan <dadachann@users.noreply.github.com>

src/langbot/pkg/persistence/alembic/versions/0020_workspace_membership_source.py

@@ -0,0 +1,49 @@
+"""add explicit Workspace membership source
+
+Revision ID: 0020_membership_source
+Revises: 001a_pgvector_dimension_3072
+Create Date: 2026-08-06
+"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = '0020_membership_source'
+down_revision = '001a_pgvector_dimension_3072'
+branch_labels = None
+depends_on = None
+
+_CONSTRAINT_NAME = 'ck_workspace_memberships_source'
+
+
+def upgrade() -> None:
+    conn = op.get_bind()
+    inspector = sa.inspect(conn)
+    if 'workspace_memberships' not in inspector.get_table_names():
+        return
+    if 'source' in {column['name'] for column in inspector.get_columns('workspace_memberships')}:
+        return
+
+    # No durable historical field distinguishes Directory-created revision-zero
+    # rows from Core invitations. Protect every existing row; production can
+    # reclassify separately after UUIDs have been verified against Space.
+    with op.batch_alter_table('workspace_memberships') as batch_op:
+        batch_op.add_column(sa.Column('source', sa.String(length=32), nullable=False, server_default='local'))
+        batch_op.create_check_constraint(
+            _CONSTRAINT_NAME,
+            "source IN ('local', 'cloud_projection')",
+        )
+
+
+def downgrade() -> None:
+    conn = op.get_bind()
+    inspector = sa.inspect(conn)
+    if 'workspace_memberships' not in inspector.get_table_names():
+        return
+    if 'source' not in {column['name'] for column in inspector.get_columns('workspace_memberships')}:
+        return
+    with op.batch_alter_table('workspace_memberships') as batch_op:
+        batch_op.drop_constraint(_CONSTRAINT_NAME, type_='check')
+        batch_op.drop_column('source')