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/cart/overview_routes.py

@@ -3,24 +3,9 @@
 
 from __future__ import annotations
 
-from quart import Blueprint, g, request
-
-from .services import get_cart_grouped_by_page
+from quart import Blueprint
 
 
 def register(url_prefix: str) -> Blueprint:
     bp = Blueprint("cart_overview", __name__, url_prefix=url_prefix)
-
-    @bp.before_request
-    async def _prepare_page_data():
-        """Load overview data for defpage route."""
-        endpoint = request.endpoint or ""
-        if not endpoint.endswith("defpage_cart_overview"):
-            return
-        from shared.sx.page import get_template_context
-        from sx.sx_components import _overview_main_panel_sx
-        page_groups = await get_cart_grouped_by_page(g.s)
-        ctx = await get_template_context()
-        g.overview_content = _overview_main_panel_sx(page_groups, ctx)
-
     return bp