fix: stream FilePond uploads to S3 and proxy private file previews

Disk-based multer plus an authenticated content endpoint match the working Node.js uploader and avoid browser signed-URL failures on SeaweedFS.

middlewares/globalErrorHandler.js

@@ -63,6 +63,13 @@ const globalErrorHandler = (err, req, res, next) => {
     }, {});
   }
 
+  // Handle Multer upload errors
+  if (err.name === 'MulterError') {
+    statusCode = 400;
+    errorCode = err.code === 'LIMIT_FILE_SIZE' ? 'VALIDATION_FAILED' : 'FILE_REQUIRED';
+    details = { reason: err.code, field: err.field };
+  }
+
   // Handle Joi validation errors if forwarded as standard Error
   if (err.isJoi) {
     statusCode = 400;