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/routes/license_routes.py

@@ -1,10 +1,11 @@
-from fastapi import APIRouter, HTTPException
-from fastapi.responses import PlainTextResponse, HTMLResponse
 from pathlib import Path
-import os
+
+from fastapi import APIRouter, HTTPException
+from fastapi.responses import PlainTextResponse
 
 router = APIRouter()
 
+
 @router.get("/licenses/lgpl.txt", response_class=PlainTextResponse)
 async def get_lgpl_license():
     """
@@ -13,6 +14,6 @@ async def get_lgpl_license():
     license_path = Path("frontend/static/licenses/lgpl.txt")
     if not license_path.exists():
         raise HTTPException(status_code=404, detail="License file not found")
-    
+
     with open(license_path, "r") as f:
         return f.read()