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>

shared/sx/pages.py

@@ -268,7 +268,7 @@ async def execute_page(
     is_htmx = is_htmx_request()
 
     if is_htmx:
-        return sx_response(oob_page_sx(
+        return sx_response(await oob_page_sx(
             oobs=oob_headers if oob_headers else "",
             filter=filter_sx,
             aside=aside_sx,
@@ -276,7 +276,7 @@ async def execute_page(
             menu=menu_sx,
         ))
     else:
-        return full_page_sx(
+        return await full_page_sx(
             tctx,
             header_rows=header_rows,
             filter=filter_sx,