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/app.py

@@ -20,6 +20,7 @@ from bp import (
     register_data,
     register_actions,
 )
+from sxc.pages import setup_blog_pages
 
 
 async def blog_context() -> dict:
@@ -80,6 +81,8 @@ async def blog_context() -> dict:
 def create_app() -> "Quart":
     from services import register_domain_services
 
+    setup_blog_pages()
+
     app = create_base_app(
         "blog",
         context_fn=blog_context,