fixed missing columns

backend/database/sql/schemas/artist_credit_artist.sql

@@ -1,5 +1,7 @@
 CREATE TABLE IF NOT EXISTS artist_credit_artist (
   id   int PRIMARY KEY,
+  artist_id int NOT NULL,
+  credit_id int NOT NULL,
   FOREIGN KEY(artist_id) REFERENCES artists(id),
   FOREIGN KEY(credit_id) REFERENCES artist_credit(id)
 );