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/celery_app.py

@@ -1,6 +1,8 @@
 # app/celery_app.py
 
 from celery import Celery
+from celery.signals import task_failure
+
 from app.config import settings
 
 celery = Celery(
@@ -14,29 +16,30 @@ celery = Celery(
 celery.conf.broker_connection_retry_on_startup = True
 
 # Set the default queue and routing so that tasks are enqueued on "document_processor"
-celery.conf.task_default_queue = 'document_processor'
+celery.conf.task_default_queue = "document_processor"
 celery.conf.task_routes = {
     "app.tasks.*": {"queue": "document_processor"},
 }
 
-# Task failure notification handler
-from celery.signals import task_failure
 
 @task_failure.connect
-def task_failure_handler(sender=None, task_id=None, exception=None, args=None,
-                        kwargs=None, traceback=None, einfo=None, **kw):
+def task_failure_handler(
+    sender=None, task_id=None, exception=None, args=None, kwargs=None, traceback=None, einfo=None, **kw
+):
     """Handler for Celery task failures to send notifications"""
-    if getattr(settings, 'notify_on_task_failure', True):
+    if getattr(settings, "notify_on_task_failure", True):
         try:
             # Import here to avoid circular imports
             from app.utils.notification import notify_celery_failure
+
             notify_celery_failure(
-                task_name=sender.name if sender else "Unknown", 
+                task_name=sender.name if sender else "Unknown",
                 task_id=task_id or "N/A",
-                exc=exception, 
-                args=args or [], 
-                kwargs=kwargs or {}
+                exc=exception,
+                args=args or [],
+                kwargs=kwargs or {},
             )
         except Exception as e:
             import logging
+
             logging.exception(f"Failed to send task failure notification: {e}")