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>

.env.demo

@@ -16,6 +16,12 @@ ALLOW_FILE_DELETE=true  # Allow deletion of file records
 PROCESSALL_THROTTLE_THRESHOLD=20  # Number of files above which throttling is applied (default: 20)
 PROCESSALL_THROTTLE_DELAY=3  # Delay in seconds between each task submission when throttling (default: 3)
 
+# **Task Retry Settings**
+# Failed tasks are automatically retried with exponential backoff and jitter.
+# TASK_RETRY_MAX_RETRIES=3          # Max retry attempts per task (default: 3)
+# TASK_RETRY_DELAYS=60,300,900      # Countdown (seconds) before each retry; 1 min, 5 min, 15 min
+# TASK_RETRY_JITTER=true            # Add ±20% random jitter to prevent thundering-herd (default: true)
+
 # **Client-Side Upload Throttling**
 # Controls pacing when the browser uploads files (especially large directory drops).
 # The browser auto-detects rate-limit (HTTP 429) responses and backs off accordingly.