fix: add migration 021 to make accounts.owner_id nullable on existing DBs

Railway already ran the old migration 020 which enforced NOT NULL on
owner_id. Since alembic won't re-run a corrected 020, this new migration
explicitly reverts the constraint for databases that already applied it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/alembic/versions/021_fix_owner_id_nullable.py

@@ -0,0 +1,30 @@
+"""fix accounts.owner_id to be nullable (circular FK with users)
+
+Revision ID: 021
+Revises: 020
+Create Date: 2026-02-07
+
+The original migration 020 enforced NOT NULL on owner_id, but this creates
+a circular FK problem: Account needs owner_id (User) and User needs
+account_id (Account). Making owner_id nullable resolves this — we create
+the Account first, then the User, then set owner_id.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '021'
+down_revision: Union[str, None] = '020'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.alter_column('accounts', 'owner_id', nullable=True)
+
+
+def downgrade() -> None:
+    op.alter_column('accounts', 'owner_id', nullable=False)