Document OpenRouter backup key setting

.env.example

@@ -10,7 +10,9 @@ GOOGLE_API_KEY=
 # GEMINI_API_KEY=
 
 # Recommended backend for all Gemini stages:
-OPENROUTER_API_KEY=
+OPENROUTER_API_KEY=
+# Optional fallback used if the primary OpenRouter key errors or returns empty text.
+OPENROUTER_API_KEY_BACKUP=
 
 # Model ids (override per run with: humeo ... --gemini-model <id>)
 GEMINI_MODEL=google/gemini-2.5-pro