Replace sx_call() with render_to_sx() across all services

Python no longer generates s-expression strings. All SX rendering now
goes through render_to_sx() which builds AST from native Python values
and evaluates via async_eval_to_sx() — no SX string literals in Python.

- Add render_to_sx()/render_to_html() infrastructure in shared/sx/helpers.py
- Add (abort status msg) IO primitive in shared/sx/primitives_io.py
- Convert all 9 services: ~650 sx_call() invocations replaced
- Convert shared helpers (root_header_sx, full_page_sx, etc.) to async
- Fix likes service import bug (likes.models → models)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cart/bp/page_admin/routes.py

@@ -49,7 +49,7 @@ def register():
         from shared.sx.page import get_template_context
         from sx.sx_components import render_cart_payments_panel
         ctx = await get_template_context()
-        html = render_cart_payments_panel(ctx)
+        html = await render_cart_payments_panel(ctx)
         return sx_response(html)
 
     return bp