Migration 15: add in_library and is_similar INTEGER columns to
explore_index. Backfills in_library from existing library MBIDs.
Search index FTS5 query now includes personalization in scoring:
ORDER BY bm25(...) - (ln(pop+1) * 1.5)
- (in_library * 3.0) - (is_similar * 1.5)
For equal text+popularity scores:
- Library artist beats unrelated by 3.0 points
- Similar artist beats unrelated by 1.5 points
- Library > Similar > Neither
Tier 3 (library) entries get in_library=1 via markInLibrary.
Tier 4 (similar) entries get is_similar=1 via markSimilar.
MB result reranking (boostWithIndexPopularity) adds a 10M
popularity bonus for library artists, ensuring they always
rank above non-library artists with equal text relevance.