style: fix all flake8 linter errors across app/ directory

- Run Black formatter and isort on all app/ files
- Remove unused imports (F401) across multiple files
- Add # noqa: F401 for intentional re-exports in celery_worker.py,
  tasks/__init__.py, utils.py, frontend.py, views/base.py
- Fix f-strings without placeholders (F541) in azure.py, notification.py,
  check_credentials.py, upload_to_onedrive.py, settings.py
- Fix bare except (E722) in upload_to_sftp.py
- Fix block comment format (E265) in models.py
- Move imports to top of file to fix E402 in celery_app.py, celery_worker.py
- Fix line-too-long (E501) by wrapping strings in multiple files
- Remove unused variable (F841) in upload_to_nextcloud.py

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

app/api/onedrive.py

@@ -2,12 +2,13 @@
 OneDrive API endpoints
 """
 
-from fastapi import APIRouter, Request, HTTPException, status, Form
 import logging
 import os
-import requests
 from datetime import datetime, timedelta
 
+import requests
+from fastapi import APIRouter, Form, HTTPException, Request, status
+
 from app.auth import require_login
 from app.config import settings
 from app.utils.oauth_helper import exchange_oauth_token