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>

account/bp/account/routes.py

@@ -56,6 +56,6 @@ def register(url_prefix="/"):
         await g.s.flush()
 
         from sx.sx_components import render_newsletter_toggle
-        return sx_response(render_newsletter_toggle(un))
+        return sx_response(await render_newsletter_toggle(un))
 
     return account_bp