Delete Bucket

This endpoint deletes a bucket and all its resources including: - S3 objects and blobs - Running Ray jobs (cancels active batch processing jobs) - Batch processing artifacts - Upload files - Unique key lookups - MongoDB metadata

The deletion is performed **asynchronously** via a background task.
Returns immediately with a task_id that can be polled via GET /v1/tasks/{task_id}.

**Response**:
- `task_id`: Use this to poll deletion status via GET /v1/tasks/{task_id}
- `status`: Initial status (PENDING)
- `bucket_id`: The bucket being deleted
- `bucket_name`: Name of the bucket
- `object_count`: Number of objects that will be deleted

**Polling**:
Poll GET /v1/tasks/{task_id} until status is COMPLETED or FAILED.
Use exponential backoff (start 1s, max 30s).