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/sx/sx_components.py

@@ -771,7 +771,7 @@ def _cart_page_admin_header_sx(ctx: dict, page_post: Any, *, oob: bool = False,
 def _cart_admin_main_panel_sx(ctx: dict) -> str:
     """Admin overview panel -- links to sub-admin pages."""
     from quart import url_for
-    payments_href = url_for("page_admin.defpage_cart_payments")
+    payments_href = url_for("defpage_cart_payments")
     return (
         '(div :id "main-panel"'
         ' (div :class "flex items-center justify-between p-3 border-b"'