Workflows: a scheduled sweep starts one run per entry, so definitions stay flat

#835 plans a `forEach` step with inner steps inside a workflow definition. D19 (`DECISIONS.md:718`) says a workflow definition stays data, and attempts are indexed by ordinal with the idempotency key `{run}-{ordinal}` and parameters snapshotted per action (`barakoCMS/Features/Workflows/WorkflowRunQueue.cs:113-119`). Inner steps per item do not fit that shape: an item's retry, idempotency and outcome have no ordinal of their own.

### Why it matters

A nightly sweep over open bookings, a reminder to every member whose subscription ends this week, or a report per class all iterate. If iteration lives inside a definition, the runner grows loops, nested attempts and partial-retry rules, and a failure on item 400 of 1000 is hard to see or resume.

### The general concept

A sweep is a trigger. It runs a saved query on a schedule and starts one ordinary run per matching entry, so each entry gets its own run, attempts, idempotency and outcome, and definitions stay flat. #835 should be amended to use this for `forEach`, and the schedule trigger in #839 is where it attaches.

### Where it lives

Core: workflows and saved queries.

### Compatibility

Not built yet, so nothing released changes. It removes the need for nested steps in the definition format, which keeps the stored definition shape as it is.

### Done when

- #835 is amended to describe iteration as a sweep trigger.
- A sweep over a query matching three entries starts three runs, and a failure in one leaves the other two succeeded.
- Rerunning the same due sweep does not start duplicate runs.

Found in the architecture sweep of 15 September 2026.

Read DECISIONS.md:718 and WorkflowRunQueue.cs:113-119 first, then inspect the workflow and saved-query entry points referenced by #835 and #839. The work is done when #835 describes iteration as a sweep trigger, a three-entry sweep creates three independent runs with isolated failure, and rerunning the due sweep creates no duplicates.

Written by the indexing model from the issue text.