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>

market/bp/browse/routes.py

@@ -29,10 +29,6 @@ def register():
     register_product(),
   )
 
-  # Mount defpage for market home (GET /)
-  from shared.sx.pages import mount_pages
-  mount_pages(browse_bp, "market", names=["market-home"])
-
   @browse_bp.get("/all/")
   @cache_page(tag="browse")
   async def browse_all():