Update app.py

app.py

@@ -28,6 +28,9 @@ if not OPENROUTER_API_KEY:
     raise RuntimeError("OPENROUTER_API_KEY environment variable not set!")
 
 EXPLAIN_MODEL = os.environ.get("EXPLAIN_MODEL")
+if not EXPLAIN_MODEL:
+    raise RuntimeError("EXPLAIN_MODEL environment variable not set!")
+    
 # ---------------------------------------------------------------------------
 # Global rate limiter for OpenRouter calls
 #