feat(tasks): add retry logic with exponential backoff and jitter

- Rewrite app/tasks/retry_config.py with compute_countdown() function
  implementing per-retry delays with ±20% jitter (default: 60s, 300s, 900s)
- Add BaseTaskWithRetry.retry() override to inject proper countdown
- Add OcrTaskWithRetry (120s, 600s, 1800s) for OCR/AI tasks
- Add UploadTaskWithRetry for cloud-storage upload tasks
- Add config settings: TASK_RETRY_MAX_RETRIES, TASK_RETRY_DELAYS, TASK_RETRY_JITTER
- Update process_with_ocr and process_with_azure tasks to use OcrTaskWithRetry
- Update all 11 upload tasks to use UploadTaskWithRetry
- Add 38 unit tests in tests/test_retry_config.py
- Update docs/ConfigurationGuide.md and .env.demo

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

app/tasks/upload_to_dropbox.py

@@ -9,7 +9,7 @@ from dropbox.exceptions import ApiError, AuthError
 
 from app.celery_app import celery
 from app.config import settings
-from app.tasks.retry_config import BaseTaskWithRetry
+from app.tasks.retry_config import UploadTaskWithRetry
 from app.utils import log_task_progress
 from app.utils.filename_utils import extract_remote_path, get_unique_filename
 
@@ -102,7 +102,7 @@ def get_dropbox_client():
         raise
 
 
-@celery.task(base=BaseTaskWithRetry, bind=True)
+@celery.task(base=UploadTaskWithRetry, bind=True)
 def upload_to_dropbox(self, file_path: str, file_id: int = None):
     """
     Upload a file to Dropbox.