Migrate all apps to defpage declarative page routes

Replace Python GET page handlers with declarative defpage definitions in .sx
files across all 8 apps (sx docs, orders, account, market, cart, federation,
events, blog). Each app now has sxc/pages/ with setup functions, layout
registrations, page helpers, and .sx defpage declarations.

Core infrastructure: add g I/O primitive, PageDef support for auth/layout/
data/content/filter/aside/menu slots, post_author auth level, and custom
layout registration. Remove ~1400 lines of render_*_page/render_*_oob
boilerplate. Update all endpoint references in routes, sx_components, and
templates to defpage_* naming.

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

blog/bp/menu_items/routes.py

@@ -23,24 +23,19 @@ def register():
         from sx.sx_components import render_menu_items_nav_oob
         return render_menu_items_nav_oob(menu_items)
 
-    @bp.get("/")
-    @require_admin
-    async def list_menu_items():
-        """List all menu items"""
+    @bp.before_request
+    async def _prepare_page_data():
+        if "defpage_" not in (request.endpoint or ""):
+            return
         menu_items = await get_all_menu_items(g.s)
-
-
         from shared.sx.page import get_template_context
-        from sx.sx_components import render_menu_items_page, render_menu_items_oob
-
+        from sx.sx_components import _menu_items_main_panel_sx
         tctx = await get_template_context()
         tctx["menu_items"] = menu_items
-        if not is_htmx_request():
-            html = await render_menu_items_page(tctx)
-            return await make_response(html)
-        else:
-            sx_src = await render_menu_items_oob(tctx)
-            return sx_response(sx_src)
+        g.menu_items_content = _menu_items_main_panel_sx(tctx)
+
+    from shared.sx.pages import mount_pages
+    mount_pages(bp, "blog", names=["menu-items-page"])
 
     @bp.get("/new/")
     @require_admin