deploy: skip --delete on R2 sync when partial rebuild

When EXISTING_VERSIONS is set, the build only produces HTML for a subset
of versions and the bucket already contains the others. s5cmd sync with
--delete would purge those existing keys. Omit --delete in that case and
keep it for full rebuilds (EXISTING_VERSIONS empty).

Assisted-by: Claude Opus 4 (Claude Code)

Jenkinsfile

@@ -141,14 +141,24 @@ docker run --rm \
                         string(credentialsId: 'r2-access-key-id', variable: 'AWS_ACCESS_KEY_ID'),
                         string(credentialsId: 'r2-secret-access-key', variable: 'AWS_SECRET_ACCESS_KEY')
                     ]) {
+                        // When EXISTING_VERSIONS is set this is a partial rebuild,
+                        // so omit --delete to preserve versions already in the bucket.
                         sh '''
+if [ -n "${EXISTING_VERSIONS}" ]; then
+  DELETE_FLAG=""
+  echo "Partial rebuild (EXISTING_VERSIONS set) -- preserving existing objects."
+else
+  DELETE_FLAG="--delete"
+  echo "Full rebuild -- bucket will be synced with --delete."
+fi
+
 docker run --rm \
   -v "$(pwd)/html:/workspace/html" \
   -e AWS_ACCESS_KEY_ID \
   -e AWS_SECRET_ACCESS_KEY \
   peakcom/s5cmd:latest \
   --endpoint-url "https://${R2_ACCOUNT_ID}.r2.cloudflarestorage.com" \
-  sync --delete /workspace/html/ "s3://${R2_BUCKET_NAME}/"
+  sync ${DELETE_FLAG} /workspace/html/ "s3://${R2_BUCKET_NAME}/"
                         '''
                     }
                 }