Hot-reload product cache after DB changes without restarting the server. Builds the new cache first, then swaps atomically so concurrent scout runs never observe an empty cache window (BUG-09).

2026-09-24: a tenant reloads only its OWN products. Any key used to rebuild the whole cache — re-embedding every tenant's anchors on the brain's OpenAI key, as often as a customer cared to call it.