Three additions to the base class, all default-inactive:
_post_view_url(post) -> str | None
Override to provide the post-view HTML URL for the per-post
category scrape path. Default None (Danbooru/e621 skip it).
_tag_api_url() -> str | None
Override to provide the batch tag DAPI base URL for the fast
path in CategoryFetcher. Default None. Only Gelbooru proper
benefits — the fetcher's probe-and-cache determines at runtime
whether the endpoint actually honors the names= parameter.
self.category_fetcher = None
Set externally by the factory (client_for_type) when db and
site_id are available. Gelbooru-shape and Moebooru clients use
it; Danbooru/e621 leave it None.
No behavior change at this commit. Existing clients inherit the
defaults and continue working identically.