Update site config to indicate if AI search is enabled

[MillenniumFalconMechanic]

Overview

Update the site config schema to support enabling/disabling AI search functionality. When enabled, the site config should also provide the concept database endpoint used to fetch facet definitions / concept mappings needed for LLM-assisted parsing.

Approach

Specify the "enabled" flag at the entity list level. [TBD with @NoopDog]