Download files beside their final path, then move them into place

A download cut short left a partial file at the path whose existence skips the download, so every later start used it. See Model Weights in HARP's deployment guide.

backend_worker.py

@@ -69,7 +69,10 @@ def _ensure_model(model_name):
         out = os.path.join(dest, fname)
         if not os.path.exists(out):
             url = f"https://storage.googleapis.com/{_GCS_BUCKET}/{urllib.parse.quote(obj, safe='')}"
-            urllib.request.urlretrieve(url, out)
+            # Fetched beside its final path and moved there once complete, so that a download
+            # cut short cannot leave a file the check above would take for the checkpoint
+            urllib.request.urlretrieve(url, out + ".part")
+            os.replace(out + ".part", out)
     if not os.path.exists(gin_file):
         raise RuntimeError(f"Downloaded {model_name!r} but operative_config-0.gin is missing.")
     return dest