Migrate 6 reactive demo handlers from Python f-strings to SX defhandlers

Moved flash-sale, settle-data, search-products/events/posts, and catalog
endpoints from bp/pages/routes.py into sx/sx/handlers/reactive-api.sx.
routes.py now contains only the SSE endpoint (async generators need Python).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/sx/handlers.py

@@ -154,7 +154,9 @@ async def execute_handler(
     if use_ocaml:
         from .ocaml_bridge import get_bridge
 
-        # Serialize handler body with bound params as a let expression
+        # Serialize handler body with bound params as a let expression.
+        # Define constants and defcomps from the handler file are available
+        # in the kernel's global env (loaded by _ensure_components).
         param_bindings = []
         for param in handler_def.params:
             val = args.get(param, args.get(param.replace("-", "_"), NIL))