Deprecate synchronous upload capability

Changed

• Breaking: dataset.append() now always uses the async pipeline and returns an AsyncJob. The asynchronous and batch_size parameters are deprecated and ignored. All uploads (local and remote) go through the async Step Function pipeline, which handles phash computation, image optimization, and NLS search indexing.
• dataset.add_items_from_dir() now returns the AsyncJob for the upload (or None when no items are found) instead of blocking. Call job.sleep_until_complete() to wait until items are queryable and to surface upload errors.

Removed

• Synchronous upload paths for images and videos. All uploads now use the async pipeline. Use job.sleep_until_complete() to block until processing finishes.
• UploadResponse class — append() now returns AsyncJob.
• construct_append_payload() and construct_append_scenes_payload() functions.
• check_all_paths_remote() function.
• The already deprecated dataset.append_scenes() method — use dataset.append() instead.
• Synchronous branches from _append_scenes() and _append_video_scenes().

Migration: replace any dataset.append(..., asynchronous=True/False) with dataset.append(...), and call job.sleep_until_complete() before reading dataset.items, creating slices, or uploading annotations/predictions.