fix: release associations and add cleanup migration

db/migrations/000005_rm_orphan_artist_releases.sql

@@ -0,0 +1,9 @@
+-- +goose Up
+DELETE FROM artist_releases ar
+WHERE NOT EXISTS (
+    SELECT 1
+    FROM artist_tracks at
+    JOIN tracks t ON at.track_id = t.id
+    WHERE at.artist_id = ar.artist_id
+      AND t.release_id = ar.release_id
+);