Program items list: scheduling checklist view (which items are scheduled and how many times)

Program managers need to verify that all approved program items have been scheduled before publishing the schedule.

## Requested behaviour

Show in the program items list (or as a separate checklist view):
- How many times each program item has been scheduled (schedule item count)
- Which items have **not** been scheduled at all, clearly highlighted

This helps catch approved items that were accidentally left unscheduled.

## Related

- #924 (schedule grid/map view)
- #926 (program items list sorting + schedule column)

## Background

Requested in customer feedback.