fix: sort same-tier artists by blended score, not original MB score

Within the same name-match tier, the US Teenagers (MB score 100)
ranked above the FR Teenagers (MB score 93) because the tiebreaker
used OriginalScore. But the FR band is globally more popular (1.3M
vs 23K listens) and has the higher blended score (82 vs 72).

Changed the within-tier tiebreaker to use the blended Score, which
already incorporates both text relevance and popularity. This ranks
the more well-known artist first among same-named exact matches.

Added OriginalScore field to MBArtist (json:"-" so it doesn't
affect the frontend) to preserve the pre-reranking MB score for
potential future use.

backend/explore/musicbrainz.go

@@ -378,6 +378,7 @@ func convertArtist(a musicbrainzws2.Artist) MBArtist {
 		Country:        string(a.CountryCode),
 		Disambiguation: a.Disambiguation,
 		Score:          a.Score,
+		OriginalScore:  a.Score,
 	}
 
 	// Extract the primary English alias when the canonical name