Atomic download of files

Prevents serving of partial downloads, especially after early-exit

src/proxpi/_cache.py

@@ -759,9 +759,11 @@ def _download_file(self, url: str, path: str):
             return
         parent, _ = os.path.split(path)
         os.makedirs(parent, exist_ok=True)
-        with open(path, "wb") as f:
+        download_path = path + ".download"
+        with open(download_path, mode="wb") as f:
             for chunk in response.iter_content(chunk_size=16 * 1024):
                 f.write(chunk)
+        os.replace(download_path, path)
         key = self._get_key(url)
         self._files[key] = _CachedFile(path, os.stat(path).st_size, 0)
         logger.debug(f"Finished downloading '{url_masked}'")