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/database.go

@@ -121,6 +121,10 @@ func (db *DB) migrate() error {
 			name: "006_add_secondary_types_to_external_releases",
 			sql:  `ALTER TABLE external_releases ADD COLUMN secondary_types TEXT;`,
 		},
+		{
+			name: "007_index_external_releases_artist_id",
+			sql:  `CREATE INDEX IF NOT EXISTS idx_external_releases_artist_id ON external_releases(artist_id);`,
+		},
 	}
 
 	for _, m := range migrations {