fix: move HEAD request out of critical path — non-blocking background thread

The 10s HEAD request for Content-Length blocked the download endpoint
response. nginx proxy in front (bench.srv2.sechpoint.app:443 → :5000)
timed out waiting, returned empty response → 'data is null' JS error.

Now wget starts immediately, response returns instantly, and HEAD
runs in a daemon thread to populate content_length for ETA later.

frontend/app.py

@@ -20,6 +20,7 @@ from logging.handlers import RotatingFileHandler
 import os
 import shutil
 import subprocess
+import threading
 import time
 import uuid
 from pathlib import Path
@@ -262,17 +263,7 @@ async def upload_raw(request: Request):
     if free_gb < 50:
         logger.warning("Low disk: %.1f GB free — download may fail", free_gb)
 
-    # Try to get file size via HEAD request (for speed estimation)
-    content_length = 0
-    try:
-        head_resp = http_requests.head(url, timeout=10, allow_redirects=True)
-        cl = head_resp.headers.get("Content-Length")
-        if cl:
-            content_length = int(cl)
-            logger.info("Download size from HEAD: %.1f GiB", content_length / (1024**3))
-    except Exception:
-        logger.info("Could not determine download size (HEAD failed — will skip ETA check)")
-
+    # Start wget immediately (HEAD request for size happens in background)
     logger.info("Starting background download: %s → %s", url, dest)
     try:
         proc = subprocess.Popen(
@@ -288,10 +279,22 @@ async def upload_raw(request: Request):
         "vmid": vmid,
         "vm_name": vm_name,
         "start_time": time.time(),
-        "content_length": content_length,
+        "content_length": 0,
         "_last_logged_bytes": 0,
     }
 
+    # Fire-and-forget HEAD request to get file size for ETA (non-blocking)
+    def _fetch_content_length():
+        try:
+            head_resp = http_requests.head(url, timeout=5, allow_redirects=True)
+            cl = head_resp.headers.get("Content-Length")
+            if cl and filename in _active_downloads:
+                _active_downloads[filename]["content_length"] = int(cl)
+                logger.info("Download size from HEAD: %.1f GiB", int(cl) / (1024**3))
+        except Exception:
+            pass
+    threading.Thread(target=_fetch_content_length, daemon=True).start()
+
     return JSONResponse({
         "phase": "downloading",
         "filename": filename,