feat: impl upload service

apps/backend/internal/ioext/counting_reader.go

@@ -0,0 +1,23 @@
+package ioext
+
+import "io"
+
+type CountingReader struct {
+	reader io.Reader
+	count  int64
+}
+
+func NewCountingReader(reader io.Reader) *CountingReader {
+	return &CountingReader{reader: reader}
+}
+
+func (r *CountingReader) Read(p []byte) (n int, err error) {
+	n, err = r.reader.Read(p)
+	r.count += int64(n)
+	return n, err
+}
+
+func (r *CountingReader) Count() int64 {
+	return r.count
+}
+