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>

test/bp/dashboard/routes.py

@@ -64,7 +64,7 @@ def register(url_prefix: str = "/") -> Blueprint:
             # S-expression wire format — sx.js renders client-side
             from shared.sx.helpers import sx_response
             from sx.sx_components import test_detail_sx
-            return sx_response(test_detail_sx(test))
+            return sx_response(await test_detail_sx(test))
 
         # Full page render (direct navigation / refresh)
         from shared.sx.page import get_template_context