fix: disable AWS SDK checksum defaults for SeaweedFS uploads

PutObject was failing with XML parse errors because flexible checksum headers are unsupported; match SeaweedFS recommended S3 client settings.

utils/s3Client.js

@@ -43,19 +43,45 @@ const buildPublicUrl = (bucketName, fileKey) => {
 
 const getS3Client = () => {
   if (!s3ClientInstance) {
+    const endpoint = String(config.S3_ENDPOINT || '').replace(/\/$/, '');
     s3ClientInstance = new S3Client({
-      endpoint: config.S3_ENDPOINT,
+      endpoint,
-      region: config.S3_REGION,
+      region: config.S3_REGION || 'us-east-1',
       credentials: {
         accessKeyId: config.S3_ACCESS_KEY,
         secretAccessKey: config.S3_SECRET_KEY
       },
-      forcePathStyle: config.S3_FORCE_PATH_STYLE
+      forcePathStyle: config.S3_FORCE_PATH_STYLE !== false,
+      // SeaweedFS / MinIO: AWS SDK v3 default flexible checksums break PutObject
+      // (XML parse / unexpected content). Only checksum when the API requires it.
+      // See: https://github.com/seaweedfs/seaweedfs/wiki/nodejs-with-Seaweed-S3
+      requestChecksumCalculation: 'WHEN_REQUIRED',
+      responseChecksumValidation: 'WHEN_REQUIRED',
+      useDualstackEndpoint: false
     });
+    logger.info(`[S3 Storage] Client ready → ${endpoint} (pathStyle=${config.S3_FORCE_PATH_STYLE !== false})`);
   }
   return s3ClientInstance;
 };
 
+const logS3Error = (label, error) => {
+  const status = error?.$metadata?.httpStatusCode;
+  const raw = error?.$response;
+  let bodyPreview = '';
+  try {
+    const body = raw?.body || raw?.reason || '';
+    if (typeof body === 'string') bodyPreview = body.slice(0, 300);
+    else if (body && typeof body.toString === 'function') bodyPreview = String(body).slice(0, 300);
+  } catch {
+    bodyPreview = '';
+  }
+  logger.error(
+    `[S3 Storage ERROR] ${label}: ${error.message}`
+    + (status ? ` (HTTP ${status})` : '')
+    + (bodyPreview ? ` body=${bodyPreview.replace(/\s+/g, ' ')}` : '')
+  );
+};
+
 const uploadToTempBucket = async (fileBuffer, filename, contentType = 'application/octet-stream') => {
   try {
     const client = getS3Client();
@@ -70,7 +96,8 @@ const uploadToTempBucket = async (fileBuffer, filename, contentType = 'applicati
     logger.info(`[S3 Storage] Uploaded temp file: ${filename} to bucket ${config.S3_TEMP_BUCKET}`);
     return { tempFileName: filename, bucket: config.S3_TEMP_BUCKET };
   } catch (error) {
-    logger.error(`[S3 Storage ERROR] Temp upload failed for ${filename}: ${error.message}`);
+    logS3Error(`Temp upload failed for ${filename}`, error);
+    // Mock fallback for test environment when S3 is unavailable
     if (config.NODE_ENV === 'test' || error.code === 'ECONNREFUSED') {
       logger.warn(`[S3 Storage MOCK] Simulated temp upload for ${filename}`);
       return { tempFileName: filename, bucket: config.S3_TEMP_BUCKET };
@@ -109,7 +136,7 @@ const commitTempFile = async (tempFilename, destinationKey = null, targetBucketK
     logger.info(`[S3 Storage] Committed ${tempFilename} → ${targetBucket}/${targetKey}`);
     return { fileKey: targetKey, bucket: targetBucket, fileUrl };
   } catch (error) {
-    logger.error(`[S3 Storage ERROR] Failed to commit temp file ${tempFilename}: ${error.message}`);
+    logS3Error(`Failed to commit temp file ${tempFilename}`, error);
     if (config.NODE_ENV === 'test' || error.code === 'ECONNREFUSED') {
       logger.warn(`[S3 Storage MOCK] Simulated file commit for ${targetKey}`);
       return {