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>

orders/sxc/pages/orders.sx

@@ -21,7 +21,7 @@
   :path "/<int:order_id>/"
   :auth :public
   :layout (:order-detail
-    :list-url (order-list-url-from-detail)
-    :detail-url (order-detail-url))
-  :filter (order-detail-filter)
-  :content (order-detail-content))
+    :list-url (order-list-url-from-detail order-id)
+    :detail-url (order-detail-url order-id))
+  :filter (order-detail-filter order-id)
+  :content (order-detail-content order-id))