Auto-mount defpages: eliminate Python route stubs across all 9 services

Defpages are now declared with absolute paths in .sx files and auto-mounted
directly on the Quart app, removing ~850 lines of blueprint mount_pages calls,
before_request hooks, and g.* wrapper boilerplate. A new page = one defpage
declaration, nothing else.

Infrastructure:
- async_eval awaits coroutine results from callable dispatch
- auto_mount_pages() mounts all registered defpages on the app
- g._defpage_ctx pattern passes helper data to layout context

Migrated: sx, account, orders, federation, cart, market, events, blog

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

cart/sxc/pages/cart.sx

@@ -7,19 +7,19 @@
   :content (overview-content))
 
 (defpage page-cart-view
-  :path "/"
+  :path "/<page_slug>/"
   :auth :public
   :layout :cart-page
   :content (page-cart-content))
 
 (defpage cart-admin
-  :path "/"
+  :path "/<page_slug>/admin/"
   :auth :admin
   :layout :cart-admin
   :content (cart-admin-content))
 
 (defpage cart-payments
-  :path "/payments/"
+  :path "/<page_slug>/admin/payments/"
   :auth :admin
   :layout (:cart-admin :selected "payments")
   :content (cart-payments-content))