Eliminate blog settings page helpers — pure .sx defpages with service data

Convert 6 blog settings pages (settings-home, cache, snippets, menu-items,
tag-groups, tag-group-edit) from Python page helpers to .sx defpages with
(service "blog-page" ...) IO primitives. Create data-driven defcomps that
handle iteration via (map ...) instead of Python loops.

Post-related page helpers (editor, post-admin/data/preview/entries/settings/edit)
remain as Python helpers — they depend on _ensure_post_data and sx_components
rendering functions that need separate conversion.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

blog/services/__init__.py

@@ -76,3 +76,6 @@ def register_domain_services() -> None:
     if not services.has("federation"):
         from shared.services.federation_impl import SqlFederationService
         services.federation = SqlFederationService()
+
+    from .blog_page import BlogPageService
+    services.register("blog_page", BlogPageService())