redesign structure, add get endpoint

internal/database/products.go

@@ -0,0 +1,14 @@
+package database
+
+import "context"
+
+func (d *Database) FindProductIDByBarcode(system string, barcode string) (id int, err error) {
+
+	ctx := context.Background()
+
+	err = d.db.QueryOne(ctx, &id, "SELECT id FROM products WHERE system = ? AND barcode = ?", system, barcode)
+	if err != nil {
+		return -1, err
+	}
+	return
+}