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/browser/app/errors.py

@@ -149,22 +149,22 @@ async def _rich_error_page(errnum: str, message: str, image: str | None = None)
         # Root header (site nav bar)
         from shared.sx.helpers import (
             root_header_sx, post_header_sx,
-            header_child_sx, full_page_sx, sx_call,
+            header_child_sx, full_page_sx, render_to_sx,
         )
-        hdr = root_header_sx(ctx)
+        hdr = await root_header_sx(ctx)
 
         # Post breadcrumb if we resolved a post
         post = (post_data or {}).get("post") or ctx.get("post") or {}
         if post.get("slug"):
             ctx["post"] = post
-            post_row = post_header_sx(ctx)
+            post_row = await post_header_sx(ctx)
             if post_row:
-                hdr = "(<> " + hdr + " " + header_child_sx(post_row) + ")"
+                hdr = "(<> " + hdr + " " + await header_child_sx(post_row) + ")"
 
         # Error content
-        error_content = sx_call("error-content", errnum=errnum, message=message, image=image)
+        error_content = await render_to_sx("error-content", errnum=errnum, message=message, image=image)
 
-        return full_page_sx(ctx, header_rows=hdr, content=error_content)
+        return await full_page_sx(ctx, header_rows=hdr, content=error_content)
     except Exception:
         current_app.logger.debug("Rich error page failed, falling back", exc_info=True)
         return None