Constant CATALOG_INDEX_TTL 

const CATALOG_INDEX_TTL: Duration;

How long an index stays fresh before a re-index is due.

This lives in Rust rather than being a frontend constant because it decides whether the local cache is authoritative — the same class of decision as include_catalog_browse, and squarely the “sync policy” the spec review checklist keeps out of the presentation layer. If it later becomes user-configurable it stays a Rust-owned setting edited through a command.