Move cover bytes to filesystem storage

backend/api_test.go

@@ -60,7 +60,7 @@ func newTestStoreURL(t *testing.T) (*store.Store, string) {
 	url := pgtest.URL(t)
 	s, err := store.Open(url, store.Owner{
 		DiscordID: testDiscordID, TokenHash: token.Hash(ownerCredential()),
-	})
+	}, t.TempDir())
 	if err != nil {
 		t.Fatalf("store.Open: %v", err)
 	}