refactor: replace download-based hash verification with lightweight size check

The previous approach downloaded the entire backup file after upload to
compute a remote SHA-256, which doubles bandwidth cost for every backup.

New approach:
- Local SHA-256 is still computed before upload (stored in record for audit)
- After upload, use provider.List() to check remote file size (single API call)
- If remote size is 0 or mismatches local size → mark failed + auto-delete
- If List() fails, log a warning but don't block (file may have uploaded fine)

This catches 0KB corrupted uploads with zero download overhead.

server/internal/backup/helpers.go

@@ -38,15 +38,6 @@ func SHA256File(path string) (string, error) {
 	return hex.EncodeToString(hash.Sum(nil)), nil
 }
 
-// SHA256Reader 计算 reader 的 SHA-256 哈希值，返回十六进制字符串
-func SHA256Reader(reader io.Reader) (string, error) {
-	hash := sha256.New()
-	if _, err := io.Copy(hash, reader); err != nil {
-		return "", fmt.Errorf("compute checksum: %w", err)
-	}
-	return hex.EncodeToString(hash.Sum(nil)), nil
-}
-
 func sanitizeFileName(value string) string {
 	builder := strings.Builder{}
 	for _, char := range strings.TrimSpace(value) {