fix: release group ranking uses blended score + artist credit matching

Two changes:

1. rerankReleaseGroups now uses blended scoring (text relevance +
   popularity) like artists, instead of pure popularity. This
   prevents obscure albums with high listen counts from outranking
   direct MB search matches.

2. boostNameMatches now uses rgMatchTier() for release groups, which
   checks artist credit before title. Albums BY the searched artist
   (tier 0: exact credit match) rank above albums that merely
   mention the artist in the title (tier 3: title substring).

   For 'hop along': Painted Shut by Hop Along → tier 0, but
   Simple Demands: A Hop Along Tribute by Various Artists → tier 3.

   Within the same tier, blended score breaks ties so more popular
   albums by the same artist rank first.

backend/explore/musicbrainz.go

@@ -441,6 +441,7 @@ func convertReleaseGroup(rg musicbrainzws2.ReleaseGroup) MBReleaseGroup {
 		SecondaryTypes:   rg.SecondaryTypes,
 		FirstReleaseDate: rg.FirstReleaseDate.String(),
 		ArtistCredit:     rg.ArtistCredit.String(),
+		Score:            rg.Score,
 	}
 }