fix: delete scheduled test plans when account is deleted

Accounts use soft-delete (setting deleted_at), so PostgreSQL's
ON DELETE CASCADE on scheduled_test_plans.account_id never fires.
Add plan deletion to the existing account deletion transaction
to ensure atomicity.

Closes Wei-Shaw/sub2api#1728

backend/internal/repository/account_repo.go

@@ -438,6 +438,9 @@ func (r *accountRepository) Delete(ctx context.Context, id int64) error {
 	if _, err := txClient.AccountGroup.Delete().Where(dbaccountgroup.AccountIDEQ(id)).Exec(ctx); err != nil {
 		return err
 	}
+	if _, err := txClient.ExecContext(ctx, "DELETE FROM scheduled_test_plans WHERE account_id = $1", id); err != nil {
+		return err
+	}
 	if _, err := txClient.Account.Delete().Where(dbaccount.IDEQ(id)).Exec(ctx); err != nil {
 		return err
 	}