feat: store keys as JWK on disk (#339)

Co-authored-by: Kyle Mendell <kmendell@ofkm.us>

backend/internal/utils/file_util.go

@@ -78,3 +78,15 @@ func SaveFile(file *multipart.FileHeader, dst string) error {
 	_, err = io.Copy(out, src)
 	return err
 }
+
+// FileExists returns true if a file exists on disk and is a regular file
+func FileExists(path string) (bool, error) {
+	s, err := os.Stat(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			err = nil
+		}
+		return false, err
+	}
+	return !s.IsDir(), nil
+}