fix: address code review findings

- Fix pagination to not trust MusicBrainz count for cutoff (prevents
  silent discography truncation / hidden missing releases)
- Add idx_external_releases_artist_id index for hot per-artist queries
- Dedupe cached_at layout constant in external_releases.go

internal/database/external_releases.go

@@ -175,11 +175,10 @@ func GetExternalReleasesByArtistWithCache(db *DB, artistID string, ttl time.Dura
 	if ttl <= 0 {
 		return nil, nil
 	}
-	// cached_at is stored in the "2006-01-02 15:04:05" UTC layout via
-	// FormatCachedAt. Compare against an explicitly formatted cutoff string in
-	// the same layout so the lexicographic comparison is a valid time ordering.
-	const layout = "2006-01-02 15:04:05"
-	cutoff := time.Now().UTC().Add(-ttl).Format(layout)
+	// cached_at is stored in utcLayout via FormatCachedAt. Compare against an
+	// explicitly formatted cutoff string in the same layout so the
+	// lexicographic comparison is a valid time ordering.
+	cutoff := time.Now().UTC().Add(-ttl).Format(utcLayout)
 	rows, err := db.Conn().Query(
 		"SELECT rgid, artist_id, title, type, release_date, is_ignored, cached_at, secondary_types FROM external_releases WHERE artist_id = ? AND cached_at >= ?",
 		artistID, cutoff,