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>

sx/app.py

@@ -54,12 +54,11 @@ def create_app() -> "Quart":
     setup_sx_pages()
 
     bp = register_pages(url_prefix="/")
-
-    from shared.sx.pages import mount_pages
-    mount_pages(bp, "sx")
-
     app.register_blueprint(bp)
 
+    from shared.sx.pages import auto_mount_pages
+    auto_mount_pages(app, "sx")
+
     return app