bug(rbac/plans): creator cannot manage own scheduled purchase created from Plans page #1442

Bug: Creator cannot manage own Scheduled Purchase created from the Plans page

Found during QA (Bugs sheet row 608, "Non-admin User" section).

Steps to reproduce

1. As a non-admin user (member of Purchaser, Standard User, Purchase Approvers, Plan Authors), create a new Plan.
2. Schedule a Purchase from that new Plan.

Expected

The Plan is created; the Scheduled Purchase is created; and the creating user can manage (e.g. cancel) that Scheduled Purchase, the same way they can cancel a Pending Purchase request they created on Purchases > Approval queue.

Actual

The user cannot manage the Scheduled Purchase they created from their own Plan.

Verify-first (important)

This may be partially covered already:

• ux(home): Standard user cannot cancel own scheduled purchases from Home page #1400 (CLOSED, PR fix(home): show Cancel button for own scheduled purchases #1421) added "cancel own scheduled purchases from the Home page".
• sec(purchases): standard user can pause/resume other users' scheduled purchases (no ownership gate) #950 (CLOSED) covered pause/resume gating of others' purchases.

PR #1421's ownership gate was scoped to the Home-page Cancel button. Before implementing, verify whether that same ownership gate already exposes management on the Plans-originated path:

• If it does, this is a stale re-test: mark the sheet row Ready-to-Test and close this as covered by fix(home): show Cancel button for own scheduled purchases #1421.
• If it does not, implement the ownership-based management gate for scheduled purchases created via the Plans page (reuse the same ownership predicate fix(home): show Cancel button for own scheduled purchases #1421 introduced, do not duplicate it).

Acceptance

• The creator of a Plans-originated scheduled purchase can cancel/manage it.
• A non-creator still cannot (ownership gate preserved).
• Regression test exercises the real Plans-page-origin path.