feat: impl upload service

apps/backend/internal/upload/err.go

@@ -0,0 +1,9 @@
+package upload
+
+import "errors"
+
+var (
+	ErrNotFound           = errors.New("not found")
+	ErrParentNotDirectory = errors.New("parent is not a directory")
+	ErrConflict           = errors.New("node conflict")
+)