fix: replace atexit with BackgroundTask for temp zip cleanup

Using atexit to clean up temporary zip files is unreliable because
atexit handlers only run when the process exits, not after each
download. This means temp files accumulate on disk, one per download,
until the server restarts.

Replace with Starlette's BackgroundTask which runs cleanup after
the response is fully sent, ensuring temp files are deleted promptly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server/routes/sessions.py

@@ -1,4 +1,3 @@
-import atexit
 import re
 import shutil
 import tempfile
@@ -6,6 +5,7 @@ from pathlib import Path
 
 from fastapi import APIRouter, HTTPException
 from fastapi.responses import FileResponse
+from starlette.background import BackgroundTask
 
 from server.settings import WARE_HOUSE_DIR
 from utils.exceptions import ResourceNotFoundError, ValidationError
@@ -64,13 +64,12 @@ async def download_session(session_id: str):
             if zip_path.exists():
                 zip_path.unlink()
 
-        atexit.register(cleanup_zip)
-
         return FileResponse(
             path=zip_path,
             filename=f"{dir_name}.zip",
             media_type="application/zip",
             headers={"Content-Disposition": f"attachment; filename={dir_name}.zip"},
+            background=BackgroundTask(cleanup_zip),
         )
     except ValidationError as exc:
         raise HTTPException(status_code=400, detail=str(exc))