The search index ready flag is an in-memory bool that resets to false
on every app restart. It was only set to true inside build(), which
runs in a goroutine after SoftScanAllLibraries completes. If the user
searched before the build goroutine started, IsReady() returned false
and the search took the slow path (LB popularity + cross-ref: ~2.3s)
even though the SQLite index had all the data from the previous build.
Now MarkReadyIfPopulated() is called eagerly in NewExploreService —
the index is queryable as soon as the service is constructed, before
any goroutines launch. If the explore_index table has rows, ready=true
immediately.