Four save call sites updated to await save_post_file (now async)
and pass category_fetcher so the template-render ensure check can
fire when needed.
_bulk_save: creates fetcher once at the top of the async closure,
shared across all posts in the batch. Probe state persists
within the batch.
_save_to_library: creates fetcher per invocation (single post).
_save_as: wrapped in an async closure (was sync before) since
save_post_file is now async. Uses bookmark_done/error signals
for status instead of direct showMessage.
_batch_download_to: creates fetcher once at the top, shared
across the batch.
New _get_category_fetcher helper returns the fetcher from a fresh
client (lightweight — shares the global httpx pool) or None if no
site is active.