When the fast path (index ready) returns no popularity for most
artists, a targeted LB ArtistPopularity POST fires for just the
missing MBIDs. This handles searches like 'shannon' where MB
returns artists not covered by the index (not sitewide top 100,
not in library, not similar to library artists).
Only fires when >50% of artists lack index data — if the index
covered most results, the backfill is skipped. Single POST call,
typically 10-30 MBIDs, goes through the LB rate limiter.
After backfill, rerankArtists runs again with the combined
popularity data, so Shannon Wright (766K listens) correctly
outranks Shannon Hale (0 listens).
yonlu
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