fix(seed): include cancel_at_period_end in test-user subscription INSERT

Discovered during Phase 9 QA: seed_test_users.py was missing the
cancel_at_period_end column in its subscriptions INSERT, but the
column is NOT NULL (added in 016_add_subscription_tables.py).
Result: seed crashed with NotNullViolationError before any users
were created, blocking auth in fresh dev environments.

Pre-existing on main; not introduced by the FlowPilot migration
branch. Default value: false.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/scripts/seed_test_users.py

@@ -161,8 +161,8 @@ async def main() -> None:
             if cfg["plan"] is not None:
                 await conn.execute(
                     text("""
-                        INSERT INTO subscriptions (id, account_id, plan, status, created_at, updated_at)
-                        VALUES (:id, :aid, :plan, 'active', :now, :now)
+                        INSERT INTO subscriptions (id, account_id, plan, status, cancel_at_period_end, created_at, updated_at)
+                        VALUES (:id, :aid, :plan, 'active', false, :now, :now)
                     """),
                     {"id": uuid.uuid4(), "aid": account_id, "plan": cfg["plan"], "now": now},
                 )