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/primitives_io.py

@@ -42,6 +42,7 @@ IO_PRIMITIVES: frozenset[str] = frozenset({
     "get-children",
     "g",
     "csrf-token",
+    "abort",
 })
 
 
@@ -328,6 +329,22 @@ async def _io_csrf_token(
     return ""
 
 
+async def _io_abort(
+    args: list[Any], kwargs: dict[str, Any], ctx: RequestContext
+) -> Any:
+    """``(abort 403 "message")`` — raise HTTP error from SX.
+
+    Allows defpages to abort with HTTP error codes for auth/ownership
+    checks without needing a Python page helper.
+    """
+    if not args:
+        raise ValueError("abort requires a status code")
+    from quart import abort
+    status = int(args[0])
+    message = str(args[1]) if len(args) > 1 else ""
+    abort(status, message)
+
+
 _IO_HANDLERS: dict[str, Any] = {
     "frag": _io_frag,
     "query": _io_query,
@@ -341,4 +358,5 @@ _IO_HANDLERS: dict[str, Any] = {
     "get-children": _io_get_children,
     "g": _io_g,
     "csrf-token": _io_csrf_token,
+    "abort": _io_abort,
 }