feat: offer a demo instance link from the live deployment

backend/app/config.py

@@ -18,6 +18,9 @@ class Settings(BaseSettings):
     model_config = SettingsConfigDict(env_file=".env", extra="ignore")
 
     app_mode: AppMode = AppMode.DEMO
+    # Optional public URL of a fixture-replay twin of this deployment; the UI
+    # offers it as an escape hatch when the live Spotify quota runs out.
+    demo_instance_url: str = ""
 
     # Spotify application and OAuth. The redirect URI must exactly match a
     # URI registered in the Spotify developer dashboard, port included.