chore(*.psql): update functions; add update_artist_key.psql for merging artists flow

queries/functions/update_album_key.psql

@@ -1,5 +1,8 @@
+CREATE OR REPLACE FUNCTION update_album_key(old_album_key TEXT, new_album_key TEXT)
+RETURNS void AS $$
 BEGIN
   UPDATE listens
   SET album_key = new_album_key
   WHERE album_key = old_album_key;
 END;
+$$ LANGUAGE plpgsql;