fix: track durations showing 0:00 — stop merging index recordings

Index recordings lack duration data (Length=0) because the explore
index only stores title/artist/popularity. When mergeIndexHits
prepended 15+ index recordings, they filled the maxResults cap and
pushed the MB recordings (which have real durations) off the list.

Removed recording merging from mergeIndexHits entirely. Index
artists and release groups are still merged (they carry popularity
data the MB results lack), but recordings don't benefit from index
merging — MB search already returns them with proper metadata.

backend/explore/explore.go

@@ -740,18 +740,11 @@ func mergeIndexHits(result *MBSearchResult, hits []SearchIndexResult) {
 		rgMBIDs[rg.MBID] = true
 	}
 
-	recMBIDs := make(map[string]bool, len(result.Recordings))
-	for _, r := range result.Recordings {
-		recMBIDs[r.MBID] = true
-	}
-
 	// Collect new entries from index.
 	var newArtists []MBArtist
 
 	var newRGs []MBReleaseGroup
 
-	var newRecs []MBRecording
-
 	for _, h := range hits {
 		switch h.EntityType {
 		case "artist":
@@ -787,16 +780,11 @@ func mergeIndexHits(result *MBSearchResult, hits []SearchIndexResult) {
 			}
 
 		case "recording":
-			if !recMBIDs[h.MBID] {
-				newRecs = append(newRecs, MBRecording{
-					MBID:         h.MBID,
-					Title:        h.Title,
-					ArtistCredit: h.ArtistName,
-					Score:        scalePopularity(h.Popularity),
-				})
-
-				recMBIDs[h.MBID] = true
-			}
+			// Skip index recordings — they lack duration data and
+			// don't add value over MB search results which have it.
+			// Index artists and release groups are still merged
+			// because they carry popularity data the MB results lack.
+			continue
 		}
 	}
 
@@ -808,10 +796,6 @@ func mergeIndexHits(result *MBSearchResult, hits []SearchIndexResult) {
 	if len(newRGs) > 0 {
 		result.ReleaseGroups = append(newRGs, result.ReleaseGroups...)
 	}
-
-	if len(newRecs) > 0 {
-		result.Recordings = append(newRecs, result.Recordings...)
-	}
 }
 
 // scalePopularity maps a raw LB listen count to a 0–100 score