Give clear error when sexp module not available

Instead of falling through to YAML parsing (which gives confusing errors),
return a clear message that artdag.sexp is required but not installed.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/routers/recipes.py

@@ -73,7 +73,9 @@ async def upload_recipe(
     variable_inputs = []
     fixed_inputs = []
 
-    if is_sexp and SEXP_AVAILABLE:
+    if is_sexp:
+        if not SEXP_AVAILABLE:
+            raise HTTPException(500, "S-expression recipes require artdag.sexp module (not installed on server)")
         # Parse S-expression
         try:
             compiled = compile_string(content)