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/bp/page_admin/routes.py

@@ -13,23 +13,6 @@ from shared.sx.helpers import sx_response
 def register():
     bp = Blueprint("page_admin", __name__)
 
-    @bp.before_request
-    async def _prepare_page_data():
-        """Pre-render admin content for defpage routes."""
-        endpoint = request.endpoint or ""
-        if request.method != "GET":
-            return
-        if endpoint.endswith("defpage_cart_admin"):
-            from shared.sx.page import get_template_context
-            from sx.sx_components import _cart_admin_main_panel_sx
-            ctx = await get_template_context()
-            g.cart_admin_content = _cart_admin_main_panel_sx(ctx)
-        elif endpoint.endswith("defpage_cart_payments"):
-            from shared.sx.page import get_template_context
-            from sx.sx_components import _cart_payments_main_panel_sx
-            ctx = await get_template_context()
-            g.cart_payments_content = _cart_payments_main_panel_sx(ctx)
-
     @bp.put("/payments/")
     @require_admin
     async def update_sumup(**kwargs):