The search index's Tier 3 (library artists) depends on MBIDs from
the artists table. If the index built before a rescan populated
those MBIDs, library artists like Flatbush Zombies wouldn't be
indexed — they're not in the sitewide top 1000 and their name
didn't match via fuzzy matching.
Fix: OnAllScansComplete hook now calls InvalidateIndexDiscographies
before StartIndexBuild. This clears the discog_built timestamp so
Tiers 2-4 re-run incrementally, picking up any new library artists
whose MBIDs were just populated by the scan.
The rebuild is incremental — only artists not already in the index
get their discographies fetched.