fix(plans): open step rows orphaned before #1046 and scheduled steps of deleted plans #1048

Follow-up from #862 / #1046.

#1046 cancels a plan's open step rows (pending, notified, paused) inside DeletePurchasePlan, so deleting a plan no longer leaves approvable plan-less rows. Two cases are left over:

1. Rows orphaned before #1046

On existing deployments, plans deleted before #1046 left their open step rows with plan_id NULL (purchase_executions.plan_id is ON DELETE SET NULL). Token approval skips requirePlanStepCap and the approve-own plan-step rule when PlanID == "", so those rows can still be approved with no cap.

A backfill needs a reliable way to tell an orphaned plan step from an ad-hoc execution. No current column does that: step_number defaults to 1 for both. Options:

• add a marker (for example source or a nullable deleted_plan_id) going forward, plus a one-off best-effort backfill;
• cancel every open plan-less row older than a cutoff (this risks canceling legitimate ad-hoc rows);
• leave them and document a manual cleanup query.

Any migration should take the next free number: 000126 is the highest on main, and the open #1020 and #1022 add 000127 and 000128.

2. scheduled rows of a deleted plan

#1046 deliberately leaves scheduled rows alone, because they were approved while the plan existed. They still fire at their scheduled time after the plan is deleted. Whether deleting a plan should also revoke its scheduled steps is a product decision.

No frontend change is implied either way.