Fix concurrent upload tests to use ORCHARD_TEST_URL env var

The concurrent upload tests were hardcoding localhost:8080 for worker
threads, but CI sets ORCHARD_TEST_URL to the actual server hostname.
This caused "Connection refused" errors in CI.

backend/tests/integration/test_upload_download_api.py

@@ -10,6 +10,7 @@ Tests cover:
 - S3 storage verification
 """
 
+import os
 import pytest
 import io
 import threading
@@ -540,7 +541,7 @@ class TestConcurrentUploads:
             try:
                 from httpx import Client
 
-                base_url = "http://localhost:8080"
+                base_url = os.environ.get("ORCHARD_TEST_URL", "http://localhost:8080")
                 with Client(base_url=base_url, timeout=30.0) as client:
                     files = {
                         "file": (