Skip Markov warmup by default

apps/theme_lab/app.py

@@ -472,7 +472,8 @@ def load_default_transformer_checkpoint() -> None:
 
 @app.on_event("startup")
 def start_background_warmup() -> None:
-    threading.Thread(target=warm_default_markov_backend, daemon=True).start()
+    if os.environ.get("THEME_LAB_WARM_MARKOV", "").lower() in {"1", "true", "yes"}:
+        threading.Thread(target=warm_default_markov_backend, daemon=True).start()
     threading.Thread(target=load_default_transformer_checkpoint, daemon=True).start()