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/sxc/pages/blog.sx

@@ -59,13 +59,14 @@
   :path "/settings/"
   :auth :admin
   :layout :blog-settings
-  :content (settings-content))
+  :content (div :class "max-w-2xl mx-auto px-4 py-6"))
 
 (defpage cache-page
   :path "/settings/cache/"
   :auth :admin
   :layout :blog-cache
-  :content (cache-content))
+  :data (service "blog-page" "cache-data")
+  :content (~blog-cache-panel :clear-url clear-url :csrf csrf))
 
 ; --- Snippets ---
 
@@ -73,7 +74,9 @@
   :path "/settings/snippets/"
   :auth :login
   :layout :blog-snippets
-  :content (snippets-content))
+  :data (service "blog-page" "snippets-data")
+  :content (~blog-snippets-content
+    :snippets snippets :is-admin is-admin :csrf csrf))
 
 ; --- Menu Items ---
 
@@ -81,7 +84,9 @@
   :path "/settings/menu_items/"
   :auth :admin
   :layout :blog-menu-items
-  :content (menu-items-content))
+  :data (service "blog-page" "menu-items-data")
+  :content (~blog-menu-items-content
+    :menu-items menu-items :new-url new-url :csrf csrf))
 
 ; --- Tag Groups ---
 
@@ -89,10 +94,16 @@
   :path "/settings/tag-groups/"
   :auth :admin
   :layout :blog-tag-groups
-  :content (tag-groups-content))
+  :data (service "blog-page" "tag-groups-data")
+  :content (~blog-tag-groups-content
+    :groups groups :unassigned-tags unassigned-tags
+    :create-url create-url :csrf csrf))
 
 (defpage tag-group-edit
   :path "/settings/tag-groups/<int:id>/"
   :auth :admin
   :layout :blog-tag-group-edit
-  :content (tag-group-edit-content id))
+  :data (service "blog-page" "tag-group-edit-data" :id id)
+  :content (~blog-tag-group-edit-content
+    :group group :all-tags all-tags
+    :save-url save-url :delete-url delete-url :csrf csrf))