List Shelf

The time shelf: sellable counts per industry × live state for businesses filed in the last 7 / 30 / 56 / 90 days and everything on file, each window's expected find upgrade per record, today's prices, and the window the run rate of new records is read from. Built at the end of every pipeline run and stored (backend/services/list_shelf.py); this reads the stored snapshot, so a buyer's request never computes — a rolled date or a partial count is rebuilt in the background.

Declared before GET /lists/{list_id} so the path is never read as a list id.