fix: add --transfers 1 --retries 5 to rclone — workaround for Proton Drive parallel upload bug

https://claude.ai/code/session_01FKCW3FDjNFj6jve4niMFXH

infra/backup.sh

@@ -140,7 +140,7 @@ log "Archive size: ${ARCHIVE_SIZE}"
 if [ -n "${BACKUP_REMOTE}" ]; then
     if command -v rclone &>/dev/null; then
         log "Uploading to ${BACKUP_REMOTE}…"
-        rclone copy "${ARCHIVE_PATH}" "${BACKUP_REMOTE}/"
+        rclone copy --transfers 1 --retries 5 "${ARCHIVE_PATH}" "${BACKUP_REMOTE}/"
         log "Upload complete."
     else
         log "WARNING: HIY_BACKUP_REMOTE is set but rclone is not installed — skipping"