Starts an on-demand re-scan of a skill inventory (the "scan now" action). Requires publisher rights on the workspace and that the inventory has opted into automatic scanning. Returns 409 (not_opted_in) when the inventory has not opted in, and 409 (indexing_in_flight, analysis_in_flight, or rescan_in_flight) when a scan or index is already running for the inventory — the operation is idempotent, so a duplicate trigger never stacks a second scan. The route only starts the workflow; the scan, persist, and follow-up indexing run inside it.