fix: reduce upload batch size to avoid 413 Request Entity Too Large

Nginx rejects the full 500-doc batch payload. Lower to 20 docs per
request to stay within the default body size limit.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ysyneu

scripts/upload.sh

@@ -27,7 +27,7 @@ SCRIPT_NAME=$(basename "$0")
 BASE_URL="${BASE_URL:-https://docs.flashcat.cloud}"
 FULL_UPLOAD="${FULL_UPLOAD:-false}"
 DRY_RUN=false
-BATCH_SIZE=500
+BATCH_SIZE=20
 
 usage() {
   cat <<EOF