fix(tasks): register upload_to_user_integration in celery_worker and add handler tests

- Add `from app.tasks.upload_to_user_integration import upload_to_user_integration`
  to app/celery_worker.py — fixes TestCeleryWorkerConfig test that enforces
  every tasks/ module is registered with Celery.

- Add tests/test_upload_handlers.py with 43 unit tests covering all 11
  per-type upload handler functions (_upload_dropbox, _upload_s3,
  _upload_google_drive, _upload_onedrive, _upload_webdav, _upload_nextcloud,
  _upload_ftp, _upload_sftp, _upload_paperless, _upload_email, _upload_rclone)
  plus 2 additional finalize_document_storage branch tests. All external
  libraries (dropbox, boto3, msal, paramiko, smtplib, subprocess, requests)
  are mocked so tests are hermetic and fast. Coverage on changed files:
  upload_to_user_integration.py 94.71%, finalize_document_storage.py 95.51%
  (both well above the 70% Codecov diff threshold).

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/celery_worker.py

@@ -38,6 +38,7 @@ from app.tasks.upload_to_onedrive import upload_to_onedrive  # noqa: F401
 from app.tasks.upload_to_paperless import upload_to_paperless  # noqa: F401
 from app.tasks.upload_to_s3 import upload_to_s3  # noqa: F401
 from app.tasks.upload_to_sftp import upload_to_sftp  # noqa: F401
+from app.tasks.upload_to_user_integration import upload_to_user_integration  # noqa: F401
 from app.tasks.upload_to_webdav import upload_to_webdav  # noqa: F401
 from app.tasks.upload_with_rclone import send_to_all_rclone_destinations, upload_with_rclone  # noqa: F401
 from app.tasks.uptime_kuma_tasks import ping_uptime_kuma  # noqa: F401