Start a follow-up investigation scoped to a clicked chart segment.

The user clicked a bar/point on the chart for page_id; we copy that page's block (its metric selection + the same dimensions) into a brand-new thread and append each explore_filters entry onto every metric selection's :explore_filters vector. The background planner then materializes the same set of charts, but every query is scoped to those filters — and to any filters the source block already carried, so drilling from within an already-drilled thread keeps the earlier scope (see metabase.explorations.query-plan.context/build-row-context). Returns immediately with the new thread stamped started_at; clients poll GET /:id for the queries to land, exactly like create.