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>

blog/bp/snippets/routes.py

@@ -47,7 +47,7 @@ def register():
 
         snippets = await _visible_snippets(g.s)
         from sx.sx_components import render_snippets_list
-        return sx_response(render_snippets_list(snippets, is_admin))
+        return sx_response(await render_snippets_list(snippets, is_admin))
 
     @bp.patch("/<int:snippet_id>/visibility/")
     @require_login
@@ -71,6 +71,6 @@ def register():
 
         snippets = await _visible_snippets(g.s)
         from sx.sx_components import render_snippets_list
-        return sx_response(render_snippets_list(snippets, True))
+        return sx_response(await render_snippets_list(snippets, True))
 
     return bp